The paper reports on a longitudinal multiple-case study that probed into four advanced university-level Chinese EFL learners' situated vocabulary learning experiences and explored the role of L2 motivation in their productive vocabulary development. In the study, Lexical Frequency Profile analysis and semi-structured interviews were conducted with the participants during the ten-month inquiry. The data showed that, after an academic year of study in the English Department, the participants were more inclined to recycle a smaller range of simple words while leaving the more difficult academic words largely unattended in productive vocabulary use. Emerging findings from an analysis of their situated learning experiences further revealed that the productive vocabulary development was mediated by multiple factors and processes within the pedagogical and sociocultural context surrounding the participants. In particular, learners' motivation regarding what they ought to do and what they wish to do guided their utilization of available learning resources and largely mediated their productive vocabulary development in this particular learning context. These findings have important pedagogical implications for instructed L2 vocabulary learning and teaching. (Contains 1 figure and 3 tables.)