The overseas immersion environment has long been considered a superior context for language learning, supposedly providing unlimited exposure to target language (TL) input and countless opportunities for authentic interaction with expert users. This article focuses on immersion programmes (IPs) for in-service language teachers--a relatively unexplored cohort--and contributes to the rather sparse research base on IPs as professional development for language teachers. It investigates perceptions teachers had of changes in their TL proficiency, the interaction opportunities encountered and how these were seen to contribute to their language learning. Findings highlighted the importance of interaction with two key factors identified by teachers as having facilitated gains in their proficiency: "opportunities to interact with native speakers" and "being immersed in the language". Interaction opportunities identified by teachers were primarily those offered by homestay families but also contact with teachers in host schools, a hitherto unreported source of interaction. Teachers also identified factors they perceived as enhancing or limiting opportunities for linguistic gain and these included both external and interpersonal factors. Programmes of short duration and speaking English instead of the TL were identified as key factors having negative impact on TL progress. Insights from this study, in particular the value of connecting teachers with host schools, can usefully inform the design of future IPs for teachers. To this end, the article concludes with suggestions that seek to ensure increased and more focused interaction opportunities, thus optimising benefits such programmes can have for language teachers and ultimately, for their students.