This study presents the results of research on the vocabulary acquisition of French students of Spanish as second language. The aim is to know (1) which of these three vocabulary-learning tasks is more effective: definition-choosing, gap-filling or sentence-writing; and (2) which kind of knowledge, receptive or productive, the participants acquire with each vocabulary-learning task. The analysis shows that the most effective task is sentence-writing, then gap-filling and, in the third place, definition-choosing. Also all the three learning tasks trained students to complete above all activities which require a receptive knowledge of the words—definition-choosing and gap-filling tasks—, but they were much less prepared to carry out the sentence-writing task, which requires a productive knowledge of the words. The only productive task proposed—sentence-writing—is the one that allowed the students to learn receptively and productively a higher number of words. We associate our results with the Involvement Load Hypothesis and Technique Feature Analysis.