A study of the viscosity rise during free radical crosslinking polymerization is presented. Techniques W.ere developed for monitoring isothermal viscosity rise during fast polymerization reactions. The chemical system used W.as composed of a dimethacrylate resin in styrene monomer, t-butyl perbenzoate initiator, and hydroquinone as the inhibitor. W.hen reduced viscosity is plotted versus conversion, data at all temperatures collapsed into a single curve. A theoretical equation is suggested for the time required to reach the gel point in free radical crosslinking polymerization W.ith inhibition. This equation shoW.ed good agreement W.ith experimental observations.