This paper explores interdisciplinary collaboration in English language teaching from a new perspective by analysing the reflections of three subject teachers. Their thoughts provide valuable insights for English language teachers when they pursue collaboration with subject teachers. The three subject teachers worked closely with the researcher on a web-based essay writing teaching project which developed an essay critiquing system and conducted a series of workshops for Cantonese English learners at a secondary school in the past few years. The workshops were well-received, with positive comments from the students. Informal interviews which resembled conversations with the subject teachers were arranged at the end of the project and their oral reflections were transcribed for analysis. The subject teachers delineated the benefits, problems, challenges and key successful factors of interdisciplinary collaboration. They felt that obtaining comments and teaching ideas from English language teachers at different stages is conducive to system design and student learning. Although their discipline encourages them to contact end-users, some practical issues such as high communication cost and publication trend are some factors that may deter collaboration.