The Wynkoop Family Papers, 1684-1930, contain correspondence, financial papers, legal papers, memorabilia, and miscellaneous material, amounting to approximately 900 items. Bulk dates are 1758 to 1779 and 1790 to 1808. Parts of the collection are in Dutch, although English predominates; occasional items are in French, German, or Spanish.

Correspondence, 1740-1930, consists of 251 letters. The periods 1761-1766 and 1790-1796 have the heaviest representation; letters of the earlier period relate to business, those of the later period to family matters.

Incoming letters to Cornelius Wynkoop (1732-circa 1807) form the principal part of the collection. There are 51 letters from his son Cornelius C. and 51 letters from his son John C. Other correspondents represented in number are John de Neufville (18 letters) and Nicolas DeRonde (7), business associates, and Jonathan Hasbrouck (12), a son-in-law. The years after 1804 are thinly represented. There are five letters written by Martin Van Buren, Jr., while touring in Europe with his father in 1853 and 1854. Martin Van Buren is represented by an A.L.S. written from Lindenwald in 1856 and by an 1857 letter to the senior Van Buren from Bradley Benedict Burt, a lawyer of Oswego, N.Y. There are three letters of Peter Erben to Abraham Bruyn Hasbrouck, a grandson of Cornelius Wynkoop (1732-circa 1807) and a president of Rutgers College. The content of correspondence after 1874 is mostly genealogical. Richard Wynkoop, compiler of three editions of the Wynkoop genealogy, and Sarah B. Reynolds, a granddaughter of Augustus Wynkoop who gathered family records, are represented. An index of the correspondence appears at the end of this inventory.

Within Financial papers, 1743-1870, most items date from 1758 to 1808 and reflect the mercantile interests of Cornelius Wynkoop (1732-circa 1807). Seven bound financial volumes span the years 1758 to 1808. A daybook of 1758-1762 also includes ledger entries of 1776-1779 at Hurley and sets of schoolmaster's records of 1773-1774 at Kingston and 1776-1777 at New York City. In a receipt book, dated 1758 but commencing two years later, are inscribed names of old New York: Beekman, Bleecker, Lefferts, Roosevelt, and Schuyler. An account book dates from the 1760s, as do record books of the brigantine Mary & Anne and the scow Friendship; these latter include daybook entries, ledgers of accounts, shipping lists, and sailing orders for voyages to Tortola and other destinations. From the Federal period are a ledger, mostly of family accounts, and a tax book for the First Ward of New York City.

A sixty-year span of business is recorded in eleven folders of accounts, including accounts for the operation of a sugarhouse, and eight folders of bills and receipts. A 1771 bill of sale for a slave, pre-Revolutionary wage records, a fragment of a proprietors' assessment list for repairs possibly at New York City, bills of exchange, bills of lading, bonds, memoranda of payment, price lists, and promissory notes are other categories of financial papers. There are two folders of miscellaneous financial records. An item of 1778 concerns the issuing of rations to Revolutionary troops on the Hudson at a time when Cornelius Wynkoop was Assistant Commissary of Issues for the Northern District.

Legal papers, 1753-1815, consist largely of papers relating to the estates of Cornelius Wynkoop's relatives and business associates.

The section of Memorabilia, 1715-1904, contains both holograph and printed material under the headings Bibles, Genealogy, and General; the last two categories are subdivided alphabetically by the name of the family or family member. Of three eighteenth-century Dutch Staten Generael Bibles in the collection, two are identifiable as Wynkoop family Bibles. In a narrative that is part of the family record in the 1715, 1727 Bible, Cornelius Wynkoop (1688- circa 1747) recounts his capture en route to Curacao by a French privateer in 1708. Genealogical material, much of it collected by Sarah B. Reynolds, includes the holograph family record, perhaps removed from a Bible now lost, of Cornelius Wynkoop (1732-circa 1807); a copy of Richard Wynkoop's Wynkoop Genealogy in the United States of America, 2nd ed. (1878 , with additions apparently in the hand of Sarah B. Reynolds; and data on allied families. The section of general memorabilia includes a lengthy religious testament of Jane Reynolds. There is a volume of Cornelius Wynkoop (1732-circa 1807) with his "Journal of a Trip to Albany," a four-page account of a trip from Kingston in July 1758 that includes an after-the-fact description of the Battle of Fort Ticonderoga; a three-page "Catalogue of Books Belonging to Corn. C. Wynkoop June 1758"; and receipts that date from 1758 to 1760. A notebook of Peter Silvester Wynkoop, apparently written while he was pastor at Ghent and Hillsdale in Columbia County, N.Y., contains baptism and marriage records, minutes of church meetings, and a record of children's Sabbath behavior. Among other items of unidentified origin are two printed lottery tickets of 1793 and 1818.

Among the Miscellaneous material, 1684-1889, are funeral lists (possibly of the funeral of John Michael Kern), an undated map of Oswego, N.Y., newspaper clippings, and newspapers. In the latter category are the New-York Gazette and General Advertiser of October 17, 1798; a reprint of the Ulster County Gazette of January 4, 1800, observing the death of George Washington; and a number of the Syracuse (N.Y.) Anti-Bonder, Satuday [sic], May 25, 1889, opposing the financing of a new Syracuse water supply by municipal bonds. The earliest item in the collection is a 1684 Hamburg imprint of eight unnumbered pages, containing an acrostic on "Hamburg vivat" and a 152-line poem with a concluding eight-line sonnet. Other poetry includes two broadsides: a New Year's address from the carriers of the Evening Post to their patrons (New York, January 1, 1803) and two copies of The carrier of the New-York Evening Post to his patrons. (January 1, 1804). Under the heading of Religious Effects are two items related to the cabala: a broadside, "Cabbalistisches Paragramma, welches auf der Wahl Ihro Wohl-Ehrwuerden Herrn Anton Ruehl, welcher den 29ten Jun. Anno 1749. zum Diacono an der St. Jacobi Kirche [in Hamburg] erwaehlet worden, verfertiget von C. D. W."; and a notebook of cabalistic tables and commentary, signed "John Brounderost, His Book 1754." There are deathbed statements of Abraham Reynolds (1819), Adrian Wynkoop (1821), and Catherine [Wynkoop?] (1829); a manuscript narrative of Christian redemption, "A memorable instance of Divine Guidance" (circa 1780?); and a poem in Dutch on the pope as Antichrist, "Vergelijking van Christus en den Roomsche paus."