The series comprises letters and postcards, 1864-1923, to the Rev. Rees Jenkin Jones from some one hundred and twenty correspondents. The contents, in addition to personal matters, relate to Unitarianism in general and in particular the cause in Aberdare and Cardiganshire; journals and periodicals, especially Yr Ymofynydd; literary matters; and biographical material.

The file comprises a scrapbook, 1895-1896, bearing the title 'Controversial', containing press cuttings, mainly from The Merthyr Express, relating chiefly to Unitarianism in Aberdare.