Grignard reactions comprise considerable hazard potentials due to the spontaneous heat release during the initiation of the exothermic reactions and the high reactivity of the Grignard compounds. To establish industrially applicable methods for an objective detection of the reaction start-up and for the accumulation of the organic halide during the process, calorimetric studies of a special Grignard reaction in a pressurised vessel were carried out using several on-line monitoring methods. In general, the process signal profiles, FTIR measurements and balance-based systems are applicable to provide the operator with additional information on the process state. Further experiments at adiabatic and isothermal conditions show significant influences of impurities (i.e., water) on the thermal process behaviour. (c) 2007 Elsevier Ltd. All rights reserved.