Series consists of business, political and personal correspondence of John Neilson, his wife Marie Neilson and sons John Neilson jr. and Samuel Neilson jr.; Laura Moorhead (Mrs. John jr.) Neilson, Mrs. John Moorhead and Marguerite Moorhead. Included among the correspondents are François-Xavier Garneau, Robert Christie, Louis-Joseph Papineau, Denis-Benjamin Viger, John Bennett and William Gowan. Some correspondence on particular subjects is to be found in other sub-series, notably the estate and business records. (volumes 1-12, 14-15, 18, 20, 28, 32-33, 35-38, 40, 42-45). Correspondence, memoranda, returns and notes, accumulated by John Neilson sr. and Dominique Mondelet during their Commission of Inquiry into penitentiary systems, 1834-1835 (under the act 3 William IV, c.10). (volume 13).
Series also consists of estate records relating to seigneuries and other properties owned by the Neilson family, including accounts, deeds and mortgages, lawsuits, inventories and some correspondence, 1666-1854. The correspondence overlaps business letters found in Series I, II and III. (volumes 16, 25, 29-31, 39 and 41). Subject files, formed of correspondence and a wide variety of documents relating to education, Indians, political affairs, agriculture and social welfare (volumes 17, 23-24 and 26); an obsolete index of correspondence (volume 34); notebooks and journals and personal account books of John Neilson (volume 19) and a poll book of Quebec County, 1827 (volume 39A).
The series also consists of 1124 postal covers covering the period from 1793 to 1858. Most postal markings are from Lower Canada and Canada East. Straight-line postmarks are well represented, with covers from Berthier (1793-1806), Montreal (1793-1828), Three Rivers (1797-1829) and York, Upper Canada (1803-1829). Other postal markings of interest are manuscript postal markings (1820-1839), double circles (1829-1850) and broken circles.