Papurau o eiddo J. Dyfnallt Owen neu a grynhowyd ganddo, 1863-1956, yn cynnwys pregethau; nodiadau bywgraffyddol ar weinidogion anghydffurfiol; llythyrau a phapurau eraill yn ymwneud ag achos yr Annibynwyr, 1863-1953; llythyrau amrywiol pellach, 1878-1956, rhai ohonynt yn cyfeirio at yr Orsedd yn Llydaw a Chernyw; dyddiaduron, 1897-1956; cyfraniadau ar gyfer eu cyhoeddi yn Y Tyst, etc., 1923-[1953]; sgriptiau radio, 1931-1955; pregethau, 1919; torion o'r wasg, [1872]-1954; dyddlyfrau taith, a nodiadau ar bynciau hanesyddol a llenyddol, 1901-1953. = Papers of and accumulated by J. Dyfnallt Owen, 1863-1956, including sermons; biographical notes on nonconformist ministers; letters and other papers concerning the Independent cause, 1863-1953; further miscellaneous letters, 1878-1956, some of which refer to the Gorsedd in Brittany and Cornwall; diaries, 1897-1956; contributions for publication in Y Tyst, etc., 1923-[1953]; radio scripts, 1931-1955; sermons, 1919; press cuttings, [1872]-1954; travel journals, and notes on historical and literary subjects, 1901-1953.

Records collected by the United Reformed Church in Wales, 1868-1973, including Wood Street English Congregational Church, Cardiff, registers, 1891-1959, and roll books, minute books and accounts, 1868-1972; St John's English Congregational Church, Flint, subscription books, 1970-1973; English Congregational Union of North Wales, minutes and other papers, 1882-1970; Flintshire District English Congregational Union, minutes and other papers, 1935-1972; Montgomeryshire English Congregational Union, minutes, 1928-1973; trust deeds and legal papers, 1948-1968; Pembrokeshire English Congregational Union, meeting attendance book, 1952-1972; and miscellaneous correspondence, printed material and addresses, 1954-1972. ; church accounts, 1930-1949 ; minute book of Wood Street Congregational Church, Cardiff, 1868-1873; attendance book of Association and executive committee meetings of the Pembrokeshire English Congregational Union, 1952-1972.

Diaries, 1910-1945, concerning his work and Capel Mair; accounts and notes relating to his work, 1907-1944; notebooks containing details of sermons, 1907-1919; carbon copies of his business letters, 1922-1941, and letters relating to Capel Mair, 1936-1938, written by Thomas Jenkins.

Letters, 1883-1928; diaries, 1923-1932; biographical materials of Welsh interest; sermons; addresses and papers on religious, moral, political and historical subjects, including Congregationalism; poetry, including hymns; and papers relating to the history of Ebenezer Welsh Congregational Church, Cardiff, and to his biography of Griffith John, missionary to China, including letters from him, 1889-1894, and proof copies of Griffith John, D.D.: Arwr China.

Records relating to the Congregational churches at Borth, Bow Street and Clarach, Cardiganshire, mainly financial records, including balance sheets, 1877-1888; register of members and their monthly contributions, 1892-1909; correspondence relating to trustees,1880-1966; and histories of the Congregational cause in Borth, [n. d.] and 1967. Further records were received in December 2009 and April 2011. These remain uncatalogued.

Papers of the Congregational Federation in Wales, 1984-1997, including correspondence, 1992-1997; minutes of various committees and council meetings, 1984-1996; papers relating to Cytun, 1992-1994; and miscellaneous bulletins and denominational papers, 1992-1996.