Keyword captions enhance intentional L2 vocabulary phonological learning and video comprehension: evidence from Japanese learners of Chinese

:Despite growing interest in captions for second language (L2) acquisition, the research on keyword captions (KC) is scarce, and their impact on L2 vocabulary phonological learning remains unexplored. This study examined whether KC provided greater benefits than full captions (FC) and no captions (NC) for intentional L2 vocabulary learning and video comprehension. A total of 131 low-proficiency Japanese learners of Chinese were randomly assigned to one of the three captioning conditions while watching an educational video, followed by tests of phonological form recognition, meaning recognition, and content comprehension. Prior to watching, participants were informed that both vocabulary and comprehension tests would follow, and were allowed to preview the test items. The results showed that learners in the KC group consistently outperformed those in the FC and NC groups across all tests, with statistically significant advantages over FC and NC in both phonological learning and comprehension. These findings offer the first empirical evidence that captions can enhance CL2 (Chinese as a second language) phonological development. The superior performance of KC is likely attributable to both its low textual density (3.3% of the full transcript), which reduced cognitive load and directed learners’ attention to essential content, and the intentional vocabulary learning context, which further enhanced learners’ engagement with target words. Overall, these results highlight the pedagogical value of well-designed, low-density keyword captions in supporting intentional vocabulary acquisition and audiovisual comprehension in CL2 instruction.