The reaction kinetics and viscosity behavior during the reaction injection molding (RIM) process of unsaturated polyester (UPE) -Polyurethane (PU) inter-penetrating polymer networks (IPNS) were studied. The interaction between the reactants of the component polymers was reflected in the reaction kinetic model. The intermediate reaction products having a certain conversion level were prepared in order to obtain the conversion dependence of viscosity. The composition dependence of blend viscosity was also evaluated for the oligomeric mixture.