An effective method for calculating molecular weight distribution in non-linear free radical copolymerization is proposed in this work. This method is based on the direct integration of a system of non-linear differential equations describing the conservation of 'dead' polymer and 'live' radicals in a reactor. A fairly general kinetic mechanism was employed to describe the complex kinetics of non-linear copolymerization. Assumptions made were justified by comparing calculated distributions with results obtained without making any assumption. This method may be applied to industrial free radical polymerization systems, thus leading to a more rational design of polymerization reactors. (C) 2003 Elsevier Ltd. All rights reserved.