The vast majority of second language (L2) vocabulary research focuses on learners' knowledge of isolated word forms. However, it is unclear to what extent this knowledge can be used as an indicator of knowledge in context (i.e. reading and listening). This study aims to shed light on this issue by comparing ESL learners' knowledge of the meaning of isolated words ("decontextual knowledge") with their knowledge of the same words in both reading and listening ("contextual knowledge"). Decontextual knowledge was measured in a free recall interview. Contextual knowledge was measured through a task in which participants paraphrased sentences containing the target items from both a written and spoken narrative. Results showed that learners' decontextual and contextual knowledge agreed in 65% of the cases. This indicates a considerable gap between the two, and emphasises that scores on decontextualised vocabulary test should not be used as predictors of learners' vocabulary knowledge in context. In addition, learners demonstrated significantly better knowledge of word meaning in the reading than listening mode, which may be due to processing difficulties in listening as well as better inferencing opportunities in reading. Two additional factors found to affect both decontextual and contextual knowledge are word frequency and learners' vocabulary size. (Contains 8 tables, 5 figures and 1 endnote.)