The papers of Sir John Morris-Jones consist mostly of correspondence. There are over 600 letters addressed to John Morris-Jones from men of letters and Celtic scholars from Wales and further afield including Dr Gwenogfryn Evans, Sir John Rhys and Sir Owen Edwards (Reference numbers 3245-3250, 32496). Also, letters written by John Morris-Jones at Oxford to William Edwards of Llanfair Pwllgwyngyll, 1885-1888 (Reference numbers 7201-7208)
Scrap books and exercise books when he was a pupil at Friars, Bangor; Christ College, Brecon and Jesus College, Oxford, 1875-1880's (Reference numbers 3251-3252, 8476-7478)
Accounts relating to the publication of Caniadau, 1906-1909 (Reference number 8480)
Extracts of the poetry of William Llyn, Sion Philip, Sion Tudur etc. with notes (Reference number 8479)
An appeal made during the First World War and published in 1914 entitled At y Cymry (Reference number 116)
A manuscript copy of A Welsh Grammar, historical and comparative, published in 1913 (Reference number 117)
Note book of gleanings compiled in the preparation of Cerdd Dafod published in 1925 (Reference number 26895)
Collections of old Welsh sayings entered for competition in Eisteddfod Meirion in 1903 when John Morris-Jones acted as adjudicator (Reference numbers 468-474, 3254-3257)
114 englynion sent to the Bangor National Eisteddfod in 1915 (Reference number 475)
Student note book of lectures delivered by John Morris-Jones at the University College of North Wales (Reference number 13942)
A note books of songs and poems by John Morris-Jones together with translations of English and German poetry (Reference number 18332-18333)
Documents relating to the Sir John Morris-Jones Memorial, 1929-1936 (Reference number 1975)
A paper by E.R. Vaughan, Bangor on the subject of John Morris-Jones. (Reference number 15441)
The collection also includes miscellaneous family papers such as a shop account book, 1852-1864; a letter written by the father of John Morris-Jones, 1863; material relating to the C.M. Chapel in Llanfair Pwllgwyngyll, 1865-1880; Llanfair Pwllgwyngyll Parish Receipt and Payment Book, 1838-1848 and Highway Rate Book, 1858-1862; note book of William Williams of Garnan Goch, Gaerwen, 1812-1832.
Further, uncatalogued material, relating to Sir John Morris-Jones is held at the Archives Department of the University of Wales Bangor.