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This study investigated the incidental acquisition of second language (L2) vocabulary through the viewing of a 30-minute science documentary and the effect of captioning on this process. Thirty-four English language learners watched two documentaries (one captioned and one uncaptioned) and were then assessed on their knowledge of a set of words appearing in the documentaries. Results of the Vocabulary Tests (Form-recall/Spelling, and Meaning recognition) showed that documentary viewing led to significant acquisition of new vocabulary and that the captioning condition had a significant effect on this learning. Captioning boosted the gains on meaning recognition, form recall and spelling. Results indicated that relative gains in the captioning condition are 8% significantly higher than in the uncaptioned condition.