Framed in a cognitive approach to task-supported L2 vocabulary learning, the present study used a pedagogical approach to investigate the relative effectiveness of tasks with different involvement loads on the vocabulary knowledge of Iranian EFL learners. The goal was to investigate the way that the construct of involvement load is related to the Input Hypothesis (Krashen, 1985) and the Output Hypothesis (Swain, 1996) to see whether the involvement load or input/output-orientation of tasks is the determining factor in task effectiveness. The participants were 127 university students from four General English classes at the Islamic Azad University, Mobarakeh Branch, who were assigned to four instructional groups. Contrary to the predictions of the Involvement Load Hypothesis (Laufer & Hulstijn, 2001), the results of the study indicated that involvement load is not the only determining factor in task effectiveness, but input/output-orientation of tasks is also a decisive parameter in task effectiveness. While Laufer and Hulstijn’s proposal is the first valuable step towards building a theory of vocabulary learning, the results of the study indicated that involvement index may well not function independently of the task type, i.e., input or output orientation of a word-focused task.