This contribution proposes an adiabatic criterion (cri(ad)) for online detection of thermal runaway region and determination of the real-time acceptable accumulation in a semibatch process. The cri(ad) is deduced based on a mathematic analysis of the mass and heat balance equations of semibatch reactors. It states that the semibatch processes are in a potential runaway situation if the value of cri(ad) is positive on a segment of the reaction path. Furthermore, the numerical simulations for semibatch second order reactions were performed under isoperibolic and isothermal operating conditions verify the validation of the criad. To take advantage of this criterion, the information on real-time temperature of the reaction mixture and conversion of the reactants are required. Specifically, the temperature of reaction mixture can be directly measured, while the conversion of the reactants can be online measured through a modified reaction calorimetry method. This criterion can potentially contribute to optimize the feeding profile as a safety constraint as well as enhance the inherent safety of the semibatch processes. (C) 2015 Elsevier B.V. All rights reserved.