Simultaneous interpenetrating polymer networks (SIPNs) of linear polyurethane (PU) and linear poly(methyl methacrylate) (PMMA) were synthesized. Kinetic studies were carried out by means of titration of -NCO functional groups for PU and gas chromatography for methyl methacrylate (MMA). Factors that influence the kinetics of SIPN formation, such as the composition ratio, temperature, initiator concentration, catalyst concentration, time of initiator addition, and presence of the chain extender 1,4-butanediol, were investigated. It was found that all of the factors that influenced the system viscosity also affected the kinetics of SIPN formation and that the step-growth polymerization process was related to the radical polymerization process. If too much of the catalyst was added to the system, SIPNs would not completely form. The kinetics of polyurethane formation obeyed a second-order function during the early period of the reaction and a third-order function at high NCO functional group conversions.