AbstractPreviously reported studies on crosslinked castable poly(dimethylsiloxanes) have been extended. A series of dimethylsiloxane difumarates were copolymerized with styrene, and some of their rheological properties were determined. Investigation of these and previously prepared polymers has shown that segregation on a microscale can take place in two‐component crosslinked systems with relatively short network chains. This may occur even if one of the components is a nonpolymeric species involved in crosslinking. Microheterogeneity was observed in systems made up of prepolymer chains of a single type, when these were short enough to expose a specific structure of the crosslinking site. It seems that microheterogeneity can be expected in crosslinked polymers when the size of the crosslinking site is significant in comparison with the network chain length and when its cohesive energy density differs considerably from that of the main network chains. Additional factors affecting microphase separation include the existence of physical interactions between chemical groups, steric positioning of interacting groups during the process of crosslinking, and physical effects of the degree of crosslinking leading to local differences in the density of the syste