<p>This research investigated whether individual differences in vocabulary knowledge influence mind wandering during first language (L1) and foreign language (L2) reading. In four independent studies involving different L1 and L2 pairings – German and English (Study 1), French and English (Study 2), English and German (Study 3), and German and French (Study 4) – participants completed measures of mind-wandering frequency, perceived reading difficulty, and comprehension. Studies 2–4 additionally assessed the intentionality of mind wandering. Across all studies, participants with lower vocabulary knowledge in their L2 compared to L1 showed higher frequencies of mind wandering during L2 compared to L1 reading. Studies 1 and 2 additionally included participants with comparable L1/L2 vocabulary knowledge and found no significant L1–L2 differences in mind wandering. Consistent with previous research, our findings suggest that the intentionality of mind wandering is sensitive to task demands. When L2 vocabulary knowledge impaired comprehension on the textbase and situation model levels, participants showed increased unintentional but not intentional mind wandering in L2 compared to L1 (Studies 3 and 4). Under these conditions, task demands likely exceeded available executive-control resources, allowing automatic and unintentional task-unrelated thoughts to intrude. In contrast, when L2 vocabulary supported comprehension at least on the textbase level (Studies 1 and 2), metacognitive evaluations became more influential. Specifically, participants showed more intentional but not unintentional mind wandering in L2 compared to L1, and perceived difficulty mediated the relationship between L2 vocabulary knowledge and mind wandering. Here, participants likely disengaged deliberately when the cognitive costs of L2 reading outweighed its perceived benefits.</p>