As an Esperantist, Tonkin has written and translated numerous works in and about the language. Between 1974 and 1980 as well as between 1986 and 1989 he was president of the Universal Esperanto Association.
As an advocate for Esperanto, Tonkin has written and translated many works related to the language. During his tenure as president of the Universal Esperanto Association, he played a significant role in promoting the language between 1974 and 1980 and again from 1986 to 1989.