This paper reports the results of an experiment designed to investigate the effectiveness of different collaborative group formations (homogeneous vs. heterogeneous in terms of prior computer knowledge levels) for learning complex tasks in computer programming from the perspective of cognitive load theory. Web-based learning platform was used to support the collaborative learning. The results of the experiment demonstrated overall benefits of the heterogeneous composition of the collaborative groups. The results also indicated that levels of learners' prior knowledge might influence the effectiveness of collaborative conditions. For learners with lower levels of prior knowledge, the heterogeneous collaborative group was more effective than homogeneous condition while there were no differences between the conditions for more advanced learners. Reported lower ratings of cognitive load and higher instructional efficiency measures for the heterogeneous condition supported the cognitive load explanation of the results.