PolyHIPE are highly porous, emulsion-templated polymers typically synthesized via free-radical polymerization within a water-in-oil (W/O) high internal phase emulsion (HIPE) whose dispersed, aqueous phase occupies more than 74% of the volume. The synthesis of a polyHIPE containing biodegradable polymers is not straightforward because the presence of both an organic phase and an aqueous phase within the HIPE limits the type of polymerization reactions that can be used. This article describes the synthesis of polyHIPE containing biodegradable poly(epsilon-caprolactone) (PCL) groups through the step-growth reaction of a diisocyanate with a flexible PCL triol to form a crosslinked polyurethane. The reaction of the diisocyanate with the water in the HIPE produced urea groups and large bubbles from the generation Of CO2. The polymer walls between these bubbles consisted of a porous, emulsion-templated structure. Polymerization with an excess of diisocyanate produced a significant enhancement in the amounts of urea and CO2. The reduction in the flexible PCL content and the enhancement in the rigid urea content produced an increase in wall modulus that was over 20-fold. The ability to synthesize polyHIPE through such step-growth reactions is an important advance in the adaptation of polyHIPE for such applications as tissue engineering. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5806-5814, 2009