Polyurethane reaction between a new polyol and TDI under isothermal conditions was investigated. The progress of the reaction was monitored by following the viscosity as well as the monomer conversion. Viscosity variations were measured by means of a cone-and-plate viscometer while conversion was estimated by IR technique. An already reported empirical relationship between these two was verified. Stockmayer’s formula for weight average molecular weight of a linearly polycondensing system was used to find molecular weights at every stage of the reaction. The interdependence of viscosity and weight average molecular weight was confirmed, and the value of the index m was obtained. © 1991, Taylor & Francis Group, LLC. All rights reserved.