Collaborative dialogue (CD) in developing L2 skills known as ‘knowledge-building dialogue’, in spite of its momentum in foreign language acquisition, has rarely been conjoined and empirically investigated with a Task-based approach. To this end, the present research was conducted with a two-fold aim: firstly, investigating the effect of task-based collaborative dialogue on English as a foreign language (EFL) learners’ speaking ability, and secondly, exploring teachers’ and learners’ perceptions of the exercised treatment process. The participants of the study consisted of 100 Iranian B.A. TEFL and Translation Studies students were identified as relatively homogeneous with regard to their language proficiency through administering Oxford Placement Test (OPT) and ten Iranian EFL teachers. The experimental group was exposed to task-based collaborative dialogues, while the control group experienced conventional mainstream teaching the speaking skill. Upon completing the treatment, CD questionnaires were administered to the experimental group of learners and teachers to investigate their perceptions of implementing CDs in speaking classes. Subsequently, the teachers and 12 students from the experimental group attended the semi-structured interview sessions to explore their views regarding the efficiency of task-based CDs. The findings indicated that the experimental group outperformed the control one regarding speaking skill. The results of the questionnaires and the semi-structured interviews complemented each other and indicated that the teachers and the learners adopted positive views toward applying the task-based CD. The findings offer some implications for the stakeholders, including material developers, EFL learners, and teachers, to incorporate task-based collaborative dialogues in second language education.
