The growing popularity of language-learning apps among language learners has highlighted the need for rigorous evaluation of their efficacy in second language acquisition (SLA). This study combined quantitative measures to comprehensively understand the effectiveness of the Busuu app in improving the productive vocabulary of Iranian intermediate EFL learners with qualitative data from in-depth interviews to explore their perceptions and experiences while using the app. Forty-three learners (17 males and 26 females with a mean age of 27.53) were selected to assess the effectiveness of the Busuu app on learners’ productive vocabulary knowledge. Among them,15 participants (all female with a mean age of 29.40) agreed to be interviewed after the Busuu study to gain insights into learners’ perceptions and experiences with app-based learning. A pretest-posttest design using the LEX30 task, a standardized productive vocabulary assessment tool, was conducted before and after the Busuu study to evaluate participants’ productive vocabulary knowledge. Paired samples t-tests revealed no significant improvement in participants’ productive vocabulary knowledge. Nonetheless, the number of responses produced in the posttest was higher than in the pretest. According to the interview data, users rated vocabulary as the most favored feature of the app, while restricted access to premium content was noted as the least favored. Most participants acknowledged that the app exceeded their initial expectations before installing it. Additionally, improving vocabulary knowledge was the users’ primary expectation from utilizing the app. Furthermore, approximately all participants found the app beneficial for language learning. However, most of them reported low levels of enjoyment during app usage. The results can contribute to understanding what needs to be done to upgrade the quality and potential of the Bussu app, how the app can better support EFL learners in effectively activating their lexical knowledge, and what the users’ experiences and expectations from a language-learning app are.
