The collection consists of letters written to Count Mensdorff-Pouilly, by members of the English royal family and by Leopold, King of the Belgians. The principal groups are: Queen Victoria to Alphonse Mensdorff-Pouilly, 68 letters 1843-1871; Mary Louisa Victoria, Duchess of Kent to Alphonse Mensdorff-Pouilly, 88 letters 1830-1856; Prince Albert to Alphonse Mensdorff-Pouilly, 18 letters 1847-1858; Leopold I, King of the Belgians, 20 letters 1851-1865; and also letters from Victoria, Princess Royal, later Empress Frederick of Germany and from Lady Augusta Stanley. The correspondence is written in German and concerns family matters and news, with some reference to the events of the day.

The Papers consist of 16 letters written by the Countess of Rochester to her grandson Edward Henry Lee, 1st Earl of Lichfield, five letters to his wife Lady Charlotte (Fitzroy) Lee, Countess of Lichfield, and one to his mother Elizabeth (Pope) Lee Bertie, Countess of Lindsey. Also in the collection is a letter to the Countess from James Bertie, 1st Earl of Abingdon, and a letter to her from a "Cosen Bowyer".

The collection consists of fifty letters written by George Charles Grantley Fitzhardinge Berkeley (1800-1881), writer and sixth son of the fifth earl of Berkeley, to Robert Folkestone Williams (1805-1872), historian and writer. The letters chiefly concern the writing of Berkeley's autobiography, and disputes related to his revelations and interpretations of events that took place during the Regency.

The correspondence in this collection covers his tenure in this office, and consists of retained copies of reports sent to Prime Minister Lord John Russell (31 letters), and the foreign ministers the Earl of Clarendon (40 letters) and Lord Stanley (37 letters).

Allsop's correspondents include: Annie Besant, Charles Bradlaugh, Samuel Taylor Coleridge, George Jacob Holyoake, Charles Lamb, Joseph Mazzinni, Robert Owen, and others.