A classical method was utilized for determination of free isocyanates to study the kinetics of a model reaction of 2,4-and 2,6-tolylene diisocyanate (TDI) with I-butanol, which was run in the reaction medium of liquid aliphatic hydrocarbons. Such conditions made it possible to find rate constants and activation energy values for the second-order reactions under isothermal conditions. Those reactions can be employed to produce isocyanate prepolymers, which make intermediates for downstream polyurethanes. The findings are commented upon against the background of earlier kinetic data that can widely be found in the research reports published. Further, part of our work utilized the findings to provide interpretation of kinetic parameters for analogous reactions, the kinetic data being found in the presence of styrene-the monomer employed when producing polyurethane-polyester interpenetrating polymer network (IPN) systems. The hypothesis was put forward that styrene and isocyanate component(s) could react at the final stages of the process when the system becomes short in the hydroxyl substrate. That hypothesis was based on disturbances observed in the reactivity of -NCO groups (in 2,4-TDI) in the reaction system studied. The rate constants and activation energy values were determined and presented for the urethanization reaction of 1-butanol. (C) 2003 Wiley Periodicals, Inc.