Molecular dynamics and morphology in the blends of a network-forming reactive polymer and an amphiphilic block copolymer were examined as a function of the advancement of chemical reactions. In the blends containing a triblock copolymer, both microscopic (domains of the order of micrometers) and nanoscopic (domains of the order of nanometers) phase separations were observed during network formation. Interestingly, only nanoscopic phase separation was found in the blends containing a diblock copolymer. The shape and the origin of these nanoscopic features were investigated by atomic force microscopy and were found to be a function of blend composition. A concept was advanced of the three-phase nanostructured morphology that begins to farm with self-assembly of one block and continues to develop during network formation in the postassembly stage. The changes in relaxation dynamics that accompany network formation were monitored by broad-band dielectric relaxation spectroscopy (DRS) and were shown to represent a signature of the morphological state of the blend. The ability of DRS to identify and deconvolute various relaxation processes during network formation and phase separation is noteworthy and should be exploited as means of monitoring and controlling the development of nanostructured morphology in these complex systems.