This paper addresses the problem of predicting the critical parameters that characterize thermal runaway in a stirred-tank batch reactor or a storage tank with wall-cooling. For first-order reactions, four parameters are required for defining criticality. For second-order reactions, a further parameter is necessary (the stoichiometric excess of a reactant). The set of parameters used here are those used by early workers in the field. Runaway and safe conditions are identified based on the pattern of temperature-conversion trajectories near full conversion. The resulting heuristics is useful for establishing runaway boundaries on the N-S parameter plane, using CAPSA, a computer software developed ad hoc for thermal hazard prognosis. The boundaries based on CAPSA successfully predict runaway in five homogeneous and pseudohomogeneous, bench-scale, sensitive reactor systems.