This study tests the claim that word learning and retention in a second language are contingent upon a task's involvement load (i.e. the amount of need, search, and evaluation it imposes), as proposed by Laufer and Hulstijn (2001). Seventy-nine beginning learners of Spanish completed one of three vocabulary learning tasks that varied in the amount of involvement (i.e. mental effort) they induced: reading comprehension (no effort), reading comprehension plus target word suppliance (moderate effort), and sentence writing (strong effort). Passive and active knowledge of the target words was assessed immediately after treatment and two weeks later. In line with the predictions of the Involvement Load Hypothesis, retention was highest in the sentence writing task, lower in the reading plus fill-in task, and lowest in the reading comprehension task. However, when time on task was considered, the benefit associated with more involving tasks faded. The results are discussed in light of form-focused vocabulary instruction. (Contains 3 tables and 11 notes.)