Online teacher professional development (oTPD) for English language teachers is of increasing interest, and so is research into teacher motivation as one potential success determinant of oTPD courses. Situated in an oTPD programme for English as a foreign language (EFL) instructors in Vietnam, this study aims to explore motivation for oTPD in a context where development is obligatory for in-service teachers, and online options appear to work best for them. Responses to semi-structured interviews were analyzed to understand nineteen participants' initial and sustaining motivations for the programme. The results showed an important role of technology in driving teachers to both start and remain in the course; a strong influence of second language motivation on teachers' learning experiences, and a challenge to the established value of collaborative learning in virtual environments. [The volume number (24) shown on the PDF is incorrect. The correct number is 23.]