This article arises from a research study that aimed to contribute to a better understanding of the essential nature of collaborative relationships between English as an Additional Language (EAL) and mainstream subject teachers at an international secondary school in Ukraine. The research focuses on how EAL teachers support EAL learners in the mainstream classroom through collaborative practices. The article draws on qualitative data from interviews and field notes with Language & Literature, Science and EAL teachers. The interviews investigated how EAL and mainstream collaboration help support EAL learners in the mainstream classroom, and which co-planning strategies most effectively encourage collaboration, as well as EAL and mainstream teachers' views and experiences about co-planning and working collaboratively. Arising from these data, discussion focuses on the disconnect between collaboration in theory and in practice. The article concludes that although effective collaborative strategies often exist, school language policies need to include scheduled collaborative planning time between EAL and the mainstream, and that professional development for mainstream teachers should be led by EAL teachers in order to foster more balanced content and language classroom teaching.