Semi-interpenetrating polymer networks, s-IPNs, were prepared from cellulose acetate-co-butyrate (CAB), still containing 46% OH groups, and diethylene glycol bis allylcarbonate (CR39) which was crosslinked by free-radical polymerization initiated by benzoyl peroxide. Microstructure of s-IPNs (CAB/CR39) of relative weight proportions between 0.2/1 and 0.411 was examined by electron microscopy (TEM and SEM). Spheroidal compact objects of several mum diameter are randomly distributed within a microporous phase in which the pore connectivity was shown by density measurements. Such a microstructure is consistent with the formation of CAB polymer dispersed phase in a poly (CR39) network continuous phase.