The morphology of vinylester (VE) and epoxy (EP) resins, and their combination (VE/EP at a ratio 1/1) was studied by atomic force microscopy (AFM). AFM scans taken on the ion-etched surface of EP showed a featureless homogeneous structure. On the other hand, VE exhibited a two-phase microgel, whereas VE/EP a two-phase interpenetrating network (IPN) structure. A vinylester-urethane hybrid resin (VEUH) showed also the formation of an IPN-like morphology. It was concluded that cross-reactions between the hydroxyl functionality of VE and additional functional groups, such as isocyanate in VEUH or epoxy in VE/EP combinations, strongly favour the IPN formation. AFM scans revealed the compaction of the IPN structure of VE/EP upon post-curing which was associated with a prominent increase in the glass transition temperature (T-g) according to dynamic-mechanical thermal analysis. (C) 2003 Kluwer Academic Publishers.