This article offers a solution to a significant problem for teachers and researchers of language learning that confounds their interpretations and expectations of test data: The apparent simplicity of tests of vocabulary knowledge masks the complexity of the constructs they claim to measure. The authors first scrutinise task elements in two widely cited productive vocabulary measures, Lex30 (Meara & Fitzpatrick, 2000) and the Lexical Frequency Profile (LFP; Laufer & Nation, 1995), to gain a more precise understanding of the relationship between test performance and learner knowledge. Next, in three empirical studies (N = 80, 80, 100) they compare second language learners’ performance on Lex30, as the static point of reference, with LFP and with two new tests designed to investigate specific elements of the vocabulary test tasks. Correlation analyses indicate systematic differences in the tests’ capacity to capture information about the quality of learners’ word knowledge and the size of their vocabulary resource. Using the findings from this empirical work, the authors formulate a model of vocabulary capture onto which test tasks can be mapped. They demonstrate how capturing key elements of the relationship between test scores and lexical competence can guide teachers and researchers in applying and interpreting vocabulary tests.