In this article, the rheokinetics of polyurethane formation and the influence of shear rate on its kinetics have been studied. Two different linear polyurethane systems with 0% and 100% hard segments are examined in a cone and plate rheometer. The isothermal increase of viscosity during polyurethane formation has been measured at different shear rates and different temperatures and is modeled with an exponential function. The molecular weight vs. time curves and the reaction kinetic constants have been obtained for various shear rates and temperatures using gel permeation chromatography (GPC). It was concluded that kinetics of polyurethane formation is enhanced as the shear rate is increased. Phase separation is found to be responsible for irregularities in the viscosity build up due to formation of polyurethane with hard segments at high conversions. POLYM. ENG. SCI. 45:279–287, 2005. © 2005 Society of Plastics Engineers.