Thermal runaway is a significant safety concern in chemical plants with exothermic reactors. Understanding the time scale on which thermal runaway occurs for these systems can be an important factor for their safe and efficient operation. On the basis of the knowledge of time to runaway, procedures can be implemented to reduce the probability of occurrence of a thermal runaway. This paper develops correlations for time to runaway for a continuous stirred tank reactor (CSTR). The system in this study involves a reactor with a single, irreversible, firstorder, exothermic reaction. Correlations for time to runaway are developed for both adiabatic and nonadiabatic reactors. These correlations are based on results obtained by simulating the reactor system for a range of values of the dimensionless parameters that define the system. The system time constant, developed using an eigenvalue analysis, is included in the time to runaway correlations. In addition, a new definition of runaway, based on the dynamic temperature profile, is developed.