Ge͞Si͑100͒ island size distributions were monitored for coverages between 3.5 and 14.0 monolayers at growth temperatures from 450 to 600 ± C. Features in these distributions are correlated with characteristic island morphologies. The mean dome cluster size increased and the onset of island dislocation was delayed as the growth temperature increased. At 600 ± C, very large hut clusters are formed. This behavior is attributed to strain-assisted alloying of the Ge clusters. Energy dispersive x-ray analysis confirms Si diffusion into the Ge clusters at 600 ± C. An atomistic elastic model supports the interpretation that alloying is driven by strain energy enhancement near the island perimeters.