Quasi-Stationary Concentration (QSC) principle was used to study radical polymerization of vinyl monomers during reaction traveling front. Based on the experimental results and numerical calculation of one-dimensional front it was shown that the reaction zone could be both narrow and wide. Full and quasi-stationary problems were solved for different sets of kinetic and thermo-physical parameters typical for vinyl monomers' radical polymerization. Polymerization proceeds practically at a constant temperature for narrow-reaction-zone case as was shown by comparing the dimensionless values of heat-wave velocities and the final temperatures. Hence, criterial conditions obtained for isothermal polymerization permit to use QSC principle accurately. (c) 2007 Elsevier Ltd. All rights reserved.