This study uses word information scores from the Medical Research Council (MRC) Psycholinguistic Database to analyse word development in the spontaneous speech data of six adult learners of English as a second language (L2) in a one-year longitudinal study. In contrast to broad measures of lexical development, such as word frequency and lexical diversity, this study analyses L2 learners' depth of word knowledge as measured by psycholinguistic values for concreteness, imagability, meaningfulness, and familiarity. Repeated measure ANOVAs yielded significant differences over time for concreteness, imagability, and meaningfulness, where the temporal intervals act as the independent variable, and the MRC values function as the dependent variables. Non-significant results were found for familiarity scores. The results provide evidence that learners' productive vocabularies become more abstract, less context dependent, and more tightly associated over time. This indicates a deeper knowledge of second language vocabulary and has important implications for how vocabulary knowledge can be measured in future studies of L2 lexical development. (Contains 3 notes and 10 tables.)