Hollow nanostructured polymer microspheres have various potential applications; however, the development of a facile method to synthesize such particles has proved challenging. In the current work, an easy-to-use, green and low-cost method adopting a heterophase polymer dispersion strategy was developed to produce mono-disperse hollow microparticles with well-defined nanostructures. When polystyrene (PS) seed microparticles prepared using modified dispersion polymerization were dispersed in an ethanol-water mixture at 70 degrees C, they transformed into hollow PS microparticles as a result of uptake of the continuous phase by the particles. Via consecutive heterophase polymer dispersion processes involving the addition of metal precursors, hollow PS microparticles covered with bimetallic nanocrystals were obtained. That is, metal nanocrystals were produced in situ from metal precursors on the polymer particles. Based on our results and observations, we propose plausible mechanisms for the formation of these hollow microparticles. The hollow PS microparticles covered with bimetallic nanocrystals exhibited high catalytic activity at a low concentration and excellent reusability when used as catalysts for the reduction of 4-nitrophenol to 4-aminophenol using sodium borohydride as the reducing agent.