High porosity, interconnected macroporous polymers prepared by the polymerization of high internal phase emulsions (polyHIPEs) stabilized by either surfactants or a combination of surfactants and hydrophobized silica particles were used as micromixers for liquids. The polyHIPEs had permeabilities between 30 and 2550 mD. The extent of micromixing was characterized by a system of two competitive parallel reactions (fourth Bourne reaction) carried out within the polyHIPEs. These competitive reactions, an acidbase neutralization and acid-catalyzed hydrolysis, proceed at very different rates such that the extent of the hydrolysis reaction is dependent on the extent of micromixing in the system, with lower hydrolysis yields indicating more effective micromixing. It was found that the hydrolysis yields were up to 50% lower in macroporous polymers compared to those in an empty tube and also up to 45% lower than those in a conventional spiral static mixer at a flow rate of 1 mL/min. Macroporous polymers with smaller pore throat sizes were found to give lower hydrolysis yields, corresponding to better micromixing; however, they did so at the expense of decreased permeabilities. However, using macroporous polymers results in a significant pressure drop across the flow cells, whereas an empty tube and conventional spiral static mixer cause a negligible pressure drop.