The fonds consists of personal papers and correspondence, 1826-1967, of members of the family of Rees Jenkin Jones, namely the Rev. John Jones (1802-1863), Mrs Anne Griffith Jones (1852-1899), Dr Goronwy Jones (1881-1945), the Rev. Simon Jones (1879-1967) and the Rev. Rees Jenkin Jones (1835-1924) himself, together with estate records, 1620-1916, of the Clettwr estate, Cardiganshire, Cilgell Isaf estate, Carmarthenshire, and of premises in Aberdare, Glamorgan. Much of the material relates to the Unitarian cause in Wales.

An early nineteenth century manuscript containing two Unitarian tracts entitled respectively 'Cyfarchiad At Aelodau yr Eglwys Sefydledig ac at Drindodiaid Protestanaidd yn gyffredinol Lle yr annogir ac y cynghorir hwynt I droi oddi wrth Gau-addoliad tri o Bersonau I addoli Yr Unig Wir Dduw' and 'Ail Gyfarchiad At Aelodau yr Eglwys Sefydledig ac at Addolwyr tri Pherson ac un Duw, O Bob Enw'.