Based on Involvement Load Hypothesis (LAUFER; HULSTIJN, 2001), current study examined the effect of involvement load and task type on vocabulary acquisition. Six classes of EFL learners were assigned to one of six experimental groups with different involvement loads, thus leaving three groups with receptive tasks and three with productive tasks. Learners read a text and completed 10 vocabulary tasks focused on the target words while time on task was controlled across groups. The knowledge of target words was tested in two post-tests. Predictably, the findings indicated that tasks with higher involvement loads were more effective for vocabulary learning than tasks with lower involvement loads. Receptive tasks were also compared with productive ones of the same load condition. Contrary to the Involvement Load Hypothesis, productive tasks were more effective than receptive ones. Results show that the time on task does not have any effect on task efficacy.