A large number of industrially important reactions with vast heat release is carried out in multiphase reactors (heterogeneously catalyzed reactors, gas-liquid reactors, reactive distillation (RD), etc.). From safety point of view, they represent the most dangerous operational units in the chemical industry. The possibility of hazard identification of such reactors by applying the HAZOP procedure using a mathematical model is discussed. The role of model complexity in safety analysis is also described. Two case studies are presented and analyzed: RD in CSTR and fixed bed catalytic reactor. (c) 2007 Elsevier Ltd. All rights reserved.