A series of blends of a specially-synthesized, diacetylene-containing polyester (PE) and a commercial poly[ethylene-co-(vinyl acetate)] (EVA) have been prepared by mixing in solution followed by coprecipitation. DSC and WAXD characterization of the two components and each of the blends, before and after compression molding at 120 degrees C under vacuum, showed that solid-state topochemical cross-polymerization of PE takes place during molding to give polydiacetylene cross-links in PE phases. WAXD also showed that the crystal structures of PE and EVA were unaffected by molding, although composition-dependent reductions in the degrees of crystallinity were evident. The morphologies of molded specimens were examined by using DMA and SEM, together with solvent extraction experiments : blends comprising 20, 40, and 80 wt % PE have continuous-discontinuous morphologies, whereas the blend comprising 60 wt % PE has a cocontinuous morphology. The use of Raman spectroscopy to study deformation in the cross-polymerized PE phase of the blends independent of overall blend deformation is described in the following paper (Macromolecules, following paper in this issue).