<![CDATA[Vocabulary mastery remains a critical challenge in English as a Foreign Language (EFL) learning, particularly among junior high school students who have limited opportunities for English exposure beyond classroom settings. Despite the fact that Duolingo has been extensively researched as a mobile-assisted language learning tool, there is still limited data regarding how junior high school students in small-urban Indonesian settings use it in the classroom. This study investigated students’ vocabulary achievement after participating in a teacher-mediated Duolingo-supported vocabulary intervention. Employing a quantitative pre-experimental one-group pre-test and post-test design, the study involved 24 ninth-grade students at a state junior high school in Palopo, Indonesia. A validated vocabulary mastery test was used to gather data, and descriptive statistics, the Shapiro-Wilk normality test, the paired-sample t-test, and Cohen's d effect size were used for analysis. The results showed a significant improvement in students’ vocabulary scores from pre-test (M = 77.91, SD = 11.31) to post-test (M = 89.58, SD = 7.50), t(23) = 7.462, p < .001, with a large effect size (d = 1.52). These findings suggest that teacher-mediated Duolingo activities may support vocabulary development through structured mobile-based practice and feedback. However, the absence of a control group and the short intervention period limit causal interpretation. This study provides contextual information about the use of gamified mobile learning in Indonesian junior high school EFL classes.]]>
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