Belonging to the interactionist perspective, the collaborative dialogue is a technique which engages learners in joint problem-solving and knowledge building. With the aim of investigating the link between this technique and vocabulary acquisition and retention, this study was conducted with 18 threshold English as a Foreign Language (EFL) learners who were randomly chosen and put in 6 groups. They were given 6 lexical-focused tasks to be completed collaboratively and their interaction was audio-recorded. The instances of lexical-based language-related episodes (LREs) were identified in the transcribed dialogues and their outcomes were coded as “correctly resolved”, “incorrectly resolved”, and “unresolved”. The frequency of the LREs was computed; it was found that the learners were able to solve the lexical problems they encountered to a very large extent. Furthermore, the analysis of LREs and the comparison of posttest and delayed posttest scores provided convincing evidence of a link between the outcomes of LREs and the learners’ vocabulary acquisition and retention, suggesting that “correctly resolved” LREs resulted in learning and retaining the target words, while “unresolved” LREs led to non-significant learning and “incorrectly resolved” LREs led to learning the wrong meaning of the vocabulary items. The findings along with the opportunities and challenges of collaborative dialogue are discussed and possible implications for language teaching are explained.
