The Hellmuth Wolff Organ Collection consists primarily of organ plans, pamphlets, photographs, books and periodicals, and personal correspondence. The majority of these documents are associated with or represent one of the fifty organs in Wolff’s opus. In addition to depicting Wolff’s instruments, the collection of organ plans also includes Wolff’s drawings for proposed but not-constructed organs, plans by other organ builders, and plans drawn by Wolff that are based on historical models and treatises by writers such as Dom Bédos. Of the photographs, most are associated with specific Wolff organ projects and represent a span date of the late 1960s to the early 2000s. Also in the photograph collection are images of historical organs in Europe, which Wolff presumably used in modelling and designing several of his instruments, as well as images associated with other organ projects by builders such as Casavant Frères, Karl Wilhelm, and Rieger, all of whom Wolff worked with before establishing his own workshop in Laval. The collection of pamphlets includes programs, program notes, and concert series information for several of the Recitals and concerts performed on Wolff’s instruments. Of the periodicals and books, titles include The American Organist, Choir & Organ, and Orgelforum. Also in the collection are several three-dimensional models, or maquettes, of Wolff organs.

The Sir William Osler Collection, distinct from the Bibliotheca, is an extensive archival holding of Osler's correspondence (including eighteen hundred original letters), daybooks, accounts, engagement books, legal documents, book invoices, membership certificates, notebooks, lectures, addresses, newspaper clippings, photographs, books with manuscript additions, and miscellaneous loose items formerly inserted into individual books in his library. The collection also contains various family papers, including correspondence of Lady Grace Revere Osler and Edward Revere Osler.

The collection contains private records and papers relating to the medical and teaching career of Maude Abbott. It consists in large part of correspondence, 1894-1920, including family correspondence with, among others, her sister Alice Abbott, 1904-1919, and her brother Rev. Harry M. Babin, 1916-1920. Also included are manuscripts and drafts of articles and addresses; case reports; post-mortem records; glass slides and drawings; exhibit panels largely pertaining to her research on congenital heart disease; programmes of medical meetings, 1902-1937; reprints and papers relating to the history of medicine in Montreal and Quebec, as well as to the history of McGill, 1829-1936. In addition, there are photographs, some poems, an autobiographical sketch and a printed copy of her Classified and Annotated Bibliography of Sir William Osler's Publications, 1939, with corrections and annotations by W. W. Francis. Fonds also includes a draft of Abbott's letter to the Dean of Faculty of Medicine, McGill University, regarding admission (1889).

The collection contains correspondence, curriculum vitae, a poem written by his wife, Elizabeth Turnor, a record of medical examinations for the Coroner's Court of Montreal, 1894 and lectures notes. The greater part of the notes consists of embryology notes made at the Anatomisches Institut, Munich, around 1900.

Three letters from Seaton relating to the Rebellion of 1837, including a letter to General Fitzroy Sommerset about the capture of St-Eustache.

Collection consists of 36 autograph letters written to James Cameron and his son Archie. The correspondence includes letters by friends, associates, and members of the Cameron family in Toronto, Lake Beauport, Scotland, England, Hull, and Montreal. Subjects include family news, freemasonry, business (regarding deed of sales), factory work in Toronto, curling, and an April Fools' Day letter. With three extant envelopes.

The collection consists of autographed signed letters from multiple senders, representing authors, artists, politicians, and other figures from Canada and Europe. The letters were accumulated by the Rare Books and Special Collections unit of the McGill Library over many years and assembled into the collection.

Consists of an English translation of a letter to the Marquis de la Jonquière written by Antoine-Louis Rouillé, comte de Jouy, secretary of state for the French Navy, at Versailles, dated 28 February 1750. The letter discusses an immediate release of prisoners of war taken during conflicts between the French and British colonies. It also includes a mention of Indigenous allies of England and France, and Indigenous people captured during the conflicts: "the Indian Prisoners among the two Nations be likewise released, but after all the French and English Prisoners are released" The letter also includes the name of examiner Josiah Willard, secretary of the province of Massachusetts-Bay.

Letters and diaries, including typescript transcriptions with photographs, newspaper clippings and ephemera.

This collection consists of translations from Terence, begun in 1778, and correspondence on military, political and private matters, 1787-1821, mostly addressed to Thomas Coutts, banker.