This study investigates EFL learners' dialogues in synchronous task-based computer-mediated communication (CMC). The focus is on whether learners engage each other in text-based dialogues regarding the language use in pursuit of the task goal in the CMC context and how their mutual engagement impacts their language learning. Sixteen Chinese tertiary-level learners voluntarily participated in this study. They were randomly assigned to eight virtual pairs, and completed four collaborative tasks via Moodle, a course management system. The study was conducted within a sociocultural framework, especially Swain's concept of collaborative dialogue. Language-related episode (LRE) was employed as a research tool to analyze the learners' dialogue concerning their language use during the completion of the tasks. The data set includes recorded online chat logs, a post-task survey that elicited the learners' perspectives on the online collaborative learning and two individualized posttests (immediate and delayed). The findings revealed that learners did assist each other in attending to language forms through collaborative dialogue, which consequently enhanced their language learning. (Contains 3 tables.)