To date, most interpenetrating polymer networks (IPNs) are developed for slow processes such as casting or coating. For industrial manufacturing, fast-reactive polymer processing is often required. Simply increasing the amount of catalyst and/or free-radical initiator shows some limitations. Also, increasing too much temperature may cause degradation or side reactions. For polyurethane/polyacrylate IPNs, more or less simultaneous formation of the two networks, with over 97% conversion was obtained after 4 to 6 min at 110 degrees C, using appropriate catalyst/initiator combinations. Depending on the relative kinetics of network formation, either one, two, or multiple transitions were found for a given composition. Kinetics of formation and phase behavior have been investigated by Fourier transform infrared spectroscopy and dynamic mechanical analysis.