A continuous current monitoring technique has been used to follow the resistivity change during the early stages of the isothermal bulk polymerization of an unsaturated polyester resin. The general form of the log (resistivity)–time curves is similar to the conversion-time curves of inhibited vinyl polymerization obtained by other techniques. An overall activation energy for the polymerization has been derived by using the postgelation rate of resistivity change as a polymerization rate index. An analysis of the data obtained from these experiments emphasizes the complexity of the reaction kinetics of the bulk polymerization. There appears to be an unusual dependence of the rate of initiation upon initiator concentration and hence because of the short kinetic chain length, of the final rate of polymerization upon initiator concentration.