A chemorheological viscosity model based on the free volume theory and a dimensionless curing time (t/t(gel)) was used to study the complex viscosity changes during the curing of thermosetting unsaturated polyester (USPE) and epoxy resins. Themodel has five parameters (C-1, C-2, C-3, N, and t(gel)) to be determined experimentally. C-1 and C-2 are material constants determined by the complex viscosity/temperature data of neat resin, and C-3 can be calculated from C-1, C-2, and the temperature. N is independent of the temperature and only depends on the type and the concentration of initiator or curing agent used. The experimental correlation for N was obtained as a function of the concentration of initiator for USPE resin. The gel time, t(gel), was expressed as a function of the temperature and the initial concentration of the initiator. It was observed that model predictions of the complex viscosity changes in both isothermal and nonisothermal curing of USPE and epoxy resins agree well with experimental results. The predicted values of the temperature and the time at the minimum complex viscosity and at the gelation point during nonisothermal curing also agreed well with experimental results.