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 knowledge levels) for learning complex materials in a blending learning environment from the perspective of cognitive load theory. 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 for the heterogeneous condition supported the cognitive load explanation of the results. © Springer International Publishing Switzerland 2015.