Gel time of a glass/epoxy prepreg, HexPly (R) 1454, was investigated by a parallel plate rheometer. The prepreg is based on dicyandiamide (DICY)-cured diglycidyl ether of bisphenol A epoxy resin system. It is found that the application of the G'-G '' crossover method for gel time determination is not suitable for this system. A new approach was proposed in which the maximum tan 8 is regarded as the gel point. This can accurately define the gel point at various temperatures. The results proved that this point is independent of the applied frequency. The activation energy for the cure reaction of the system was deter-mined via gel time determination of the prepreg at different isothermal temperatures and found to be 75.0 +/- 10.2 kJ/ mol. This is in good agreement with the activation energy obtained from the dynamic DSC studies. The steady-shear rheology experiment was used to study the viscosity profile and subsequently to determine the gel point and verify the new approach for gel time determination. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1483-1489, 2011