Crystallization of one component in a polymer blend with homogeneously mixed melt causes an excess of the other in the surroundings of the crystal growth front and a general change of the melt composition with progressing crystallization. A number of different effects can arise in dependence on the ratio α = DIG² between the chain diffusion rate as expressed by the chain diffusion constant D and the spherulite growth rate G, and on the phase diagram of the blend. α, in its turn, can be controlled by the crystallization temperature T_c. Composition profiles can develop in the spherulite surroundings at intermediate T_c’s. These profiles can have a width of more than l00µm. They can be imagined by suitable experimental means; their evaluation allows the determination of D which turn out to be of the order l00µm²/h. In systems with miscibility gap, crystallization induced phase separation which starts at the spherulite surface can occur. At low T_c`s, the amount of the amorphous component as released by crystallization of the other can be included in the growing crystalline aggregates, this giving rise to the development of spherulites with rather irregular internal and dendritic habit. The at first local and later global change in composition is accompanied with corresponding changes in the glass temperature which in turn affect the crystallization rate too. In the paper, both the thermodynamics and the structural consequences of the crystallization induced composition changes are demonstrated and discussed in detail.