The structures of in situ generated clusters and the level of physical interactions in two types of networks differing from the chemistry of crosslinking were studied by means of small-angle X-ray analysis and dynamic mechanical spectroscopy. For the first type of networks, the crosslinks result from the hydrolysis and condensation of ethoxysilane endgroups, thus generating silicon-rich dispersed phase. In the second case, the crosslinks result from the formation of urethane units by introducing a triol. In the two cases, different types of soft segment precursors having different polarities are considered. alpha,omega-hydroxyl-terminated oligomers of hydrogenated polybutadiene or polycaprolactone or a polyester from oleic acid are used. The miscibility of the soft-segment chains with the relatively polar crosslinks is the most important parameter for understanding the morphology and the mechanical behavior of such materials. The main difference obtained from the SAXS analysis and DMS experiments is that the silicon-rich clusters appear to be stiffer and well, separated in comparison with the trimethylolpropane-urethane crosslinks. In addition, in the case of silica clusters generated in situ, the phase separation plays an important role. In the polycaprolactone-based systems, the formation of clusters is mainly governed by the nature and the reactivities of the functional groups. As a consequence, the clusters are more fractallike. The mechanical behavior, i.e., the mechanical losses and the high-temperature behavior, is discussed as a function of the existing interactions and the concentration of elastically active network chains in the different types of networks considered.