This study explores the differential effects of captions and subtitles on extensive TV viewing comprehension by adolescent beginner foreign language learners, and how their comprehension is affected by factors related to the learner, preteaching of target vocabulary, the lexical coverage of the episodes, and the testing instruments. Four classes of secondary school students took part in an 8-month intervention viewing 24 episodes of a TV series, two classes with captions, and two with subtitles. One class in each language condition received explicit instruction on target vocabulary. Comprehension was assessed through multiple-choice and true-false items, which included a combination of textually explicit and inferential items. Results showed a significant advantage of subtitles over captions for content comprehension, and prior vocabulary knowledge emerged as a significant predictor--particularly in the captions condition. Comprehension scores were also mediated by test-related factors, with true-false items receiving overall more correct responses while textually explicit and inferential items scores differed according to language of the on-screen text. Lexical coverage also emerged as a significant predictor of comprehension.