This work demonstrates the extension of the runaway criteria originally developed by Van Welsenaere and Froment and the more recent divergence criterion to the "short-stopping" of batchwise free-radical polymerization processes for mitigating thermal runaway via injection of free-radical scavengers in response to early detection of imminent runaway. A transient, nonisothermal, well-mixed batch reactor model is developed for free-radical solution polymerization of methyl methacrylate (MMA) as an illustrative example, with instantaneous addition of inhibitor occurring at predetermined injection or response times. Three parameters of the "short-stopping" mitigation procedure are investigated, specifically the injection time, inhibitor quantity, and inhibitor activity. Model results are analyzed in terms of Semenov number (psi) and critical Semenov number (psi(c)) trajectories in time. Results indicate the general criterion for a sufficient "short-stopping" recipe is to ensure a negative time derivative of the local Semenov number upon inhibitor injection (i.e., d psi/dt vertical bar(t=tinj),= < 0), as opposed to the far more conservative criteria of reducing the local Seminov number to below the corresponding local critical Seminov number (i.e. psi(t) < psi(c)(t)) at the time of injection. The proposed criterion is also compared with a similarly modified first-derivative divergence criterion and exhibits better performance than the latter.