One dimension of knowing a word is recognizing its structure. Previous research, mostly on L2 English, has revealed a close link between derivational knowledge (DK) and vocabulary size. Nevertheless, the degree of the relationship as well as the effect of the learner’s L1 vary among these studies. The present study analyzes the development of DK in L1 English learners of L2 Spanish (n = 209) taking university level courses from second to seventh semester at a US institution. The measurements explore the complex nature of DK – i.e., relational, syntactic and distributional knowledge – in both receptive and productive mode. Results showed that proficiency rather than vocabulary size, i.e., knowledge of monomorphemic words, contributes to DK. While a full receptive knowledge of some suffixes can be acquired relatively early, this is not the case for productive knowledge. This study also provides some tools to measure DK without resorting to metalanguage.