Fiber-reinforced epoxy prepregs are commonly processed using isothermal curing. Although processing appears simple, the development of properties during curing is a complex multistep process. In addition, the relationship between the curing temperature and time to a specific conversion value is not linear. Industrial processing control can be simplified using a time-temperature-transformation (TTT) diagram. However, construction of a TTT diagram requires knowledge of the rate and degree of conversion as well as the time to reach gelation and vitrification at each isotherm. The rate and degree of conversion are determined either with a series of DSC isotherms which show the change of enthalpy in time or with the change in glass transition temperature (Tg) during the isothermal cure. Since gelation appears as a change in physical properties, it is conveniently determined by DMA. Vitrification is a rubber-to-glass transition and appears in both DMA and DSC. Although vitrification is commonly determined by DMA, DSC offers increased temperature accuracy and control. However, since vitrification occurs before complete conversion, the Tg is usually masked by the curing exotherm in traditional DSC. Since vitrification and curing are thermodynamically different effects, the two signals can be separated using either Temperature Modulated DSC (TMDSC) or Step Scan DSC.