Salmau yr Eglwys yn yr Anialwch (1812)
Unitarianism in south Wales, and composed a vast corpus of hymns for Unitarian congregations. Of some three thousand hymns, only a fraction were published in Salmau yr Eglwys yn yr Anialwch (1812) and subsequent editions.
In general, the hymns are uninspiring and are heavily moralistic and theological in tone. Iolo also composed the tunes to many of his hymns.
Iolo also translated a primer for the Unitarian cause, Edward Williams, Holiadur, neu Addysgiadau Cyffredin, Hawl ac Atteb, yn Athrawiaethau Dyledswyddau Crefydd (Merthyr Tudfil, 1814).