Mixing an inhibitor to neutralize a runaway reaction is known as shortstopping. The conventional approach of using a completely mixed flow (CMF) model is inadequate for developing satisfactory operating protocols to prevent runaway reactions. In the present work, we use a computational fluid dynamics (CFD) based model to understand the role of imperfect mixing on shortstopping of a runaway reaction in a fully baffled stirred reactor. A multiple reference frame (MRF) approach is used to simulate the flows generated by a standard Rushton turbine in a stirred vessel. The computational model is then extended to simulate the simultaneous runaway and inhibition reactions. Laminar volumetric reactions are modeled by using a user-defined function. The computational model is solved using FLUENT 6.2 (of Fluent Inc., USA). The model predictions are used to understand the local runaway and quenching of runaway reactions in a vessel under the conditions of imperfect mixing. Influence of delayed addition of the inhibitor, location of addition (including multiple locations), and quantity of inhibitor added are used to study the shortstopping performance. The computational model and the results discussed in this work are useful for understanding the effect of the mixing process on the inhibition process and for developing operating protocols for preventing runaways in stirred reactors. (C) 2004 Elsevier Ltd. All rights reserved.