Hulstijn and Laufer (2001) proposed a motivational-cognitive construct of task-induced involvement to account for variation in effectiveness among different vocabulary learning tasks. Building upon their original research, this study consisted of two experiments investigating the involvement load hypothesis in vocabulary learning. Experiment 1 compared the performance of 64 adult English as a second language (ESL) learners from a range of countries at two different proficiency levels (i.e., matriculated undergraduate students vs. students in an Intensive English Program) to ascertain the effectiveness of three vocabulary tasks with different levels of task-induced involvement. Experiment 2 investigated whether two tasks hypothesized to represent the same level of task-induced involvement would result in equivalent initial learning and retention of target words by 20 adult ESL learners at two different levels of proficiency. The results of Experiment 1 showed that a higher level of learner involvement during the task promoted more effective initial vocabulary learning and better retention of the new words. The findings of Experiment 2 indicated that when different tasks had the same involvement load, they resulted in similar amounts of initial vocabulary learning and retention of new words. The results of the two experiments are discussed in light of the involvement load hypothesis. DOI: 10.1111/j.1467-9922.2008.00442.x