The annealing effects on the composition distribution of Ge islands on Si (001) were investigated by atomic force microscopy combined with selective wet chemical etching. Experimental results demonstrate that there is a strong composition redistribution occurring during island growth and postgrowth annealing. We observe that, once Ge superdomes appear, the asymmetric composition profile of the Ge domes transforms into an almost symmetric structure. Moreover, the Ge superdomes exhibit a double-ring composition profile of Si after long-time annealing. These phenomena could be explained within a simple thermodynamic model that involves only surface diffusion process.