The main objective of this work is developing a simple viscometrical method for in-line monitoring and control of weight average molecular weight ((M) over bar (w)) in solution polymerizations. The method is based on the evaluation of the approximate intrinsic viscosity of the polymer solution, at each sampling time, using a single measurement of the flow time of a diluted polymer solution through a capillary tube. Experiments were carried out for peroxide initiated styrene solution polymerization reactions in a tubular reactor. Results obtained for approximate intrinsic viscosities are in agreement with the intrinsic viscosity values published in the literature and allow fast and fair in-line values for (M) over bar (w). (C) 2001 Elsevier Science Ltd. Ail rights reserved.