Despite a growing amount of research on collaborative dialogue during task performance, little attention has been given to the occurrence of collaborative dialogue across task modality and its impact on language development. To expand our current understanding of task modality effects on learning opportunities, we examined the process of engaging in collaborative dialogue, operationalized as language-related episodes (LREs), during the oral and written modalities of two decision-making tasks and subsequent language development. Five dyads composed of Spanish (L1) -- English (L2) bilingual students learning French (L3) completed two decision-making tasks, each containing oral and written components. LREs were coded in terms of type (lexis-based, form-based), resolution (learner-learner correctly resolved, learner-teacher correctly resolved, incorrectly resolved, or unresolved), and first language use. Tailor-made posttests were created based on LREs to measure language development. Findings indicate a higher percentage of lexis-based LREs, which were more salient during the oral modality. Accuracy scores on posttests suggest that LREs promote target language development.