The purpose of this research study was to investigate the roles of English teachers and subject teachers engaged in the collaborative process of interdisciplinary teaching in English for Specific Purposes subjects at a Thai university and explore the benefits and drawbacks of implementing such collaborations. In addition, students' attitudes towards interdisciplinary teacher collaboration (ITC) in ESP classrooms were explored. Participants were English teachers, subject teachers, and students studying on ESP subjects. This research study used a mixed methods approach from four sources of data. The findings revealed the extensive roles taken on by both teachers involved in the ITCs. Roles for the English teacher involved being a lesson planner, teacher, learning organizer, and class activities designer. The subject teacher's role was identified as a consultant or informant, supporter, monitor, and facilitator. The benefits were that an English teacher gained confidence, reduced worry in teaching ESP subjects, and received instant feedback from the subject teacher. The drawbacks were that it was challenging to balance the different schedules of both teachers and that lesson planning was time consuming. Students showed positive attitudes towards this method of teaching. They liked to study because of the enjoyable and knowledgeable activities and the teacher's confidence.