The reaction of addition of ethylene glycol to diisocyanates was studied with the B3LYP method to gain an insight into the mechanism of polyurethane synthesis. It was found that the functional basis set should contain at least one diffusion function for the simulation in order to properly describe thermo-chemical features of the model reaction. Using the B3LYP/6-31+G** level the values of Gibbs free energy were estimated for the addition reaction of ethylene glycol to toluene-2,4-diisocyanate, toluene-2,6-diisocyanate, diisocyanate-[5-isocyanato-1-(isocyanatomethyl)-1,3,3-trimethylcyclohexane], 1,6-diisocyanatohexane, 4,4′-methylenebis(cyclohexyl isocyanate) and methylenebis(phenyl isocyanate). Both the gaseous phase and the benzene environment were taken into consideration. Spontaneity of the reaction proved to be dependent on both substrate type and product isomeric configuration. The trans-urethane isomer has been found to be a more stable product. Considering the values of activation energy the minor dependence on the structure of diisocyanate was observed. This confirmed Flory's postulate to be valid for the polyurethane synthesis. The highest value of activation energy was found for the first stage, which consists of ethylene glycol approach and simultaneous proton transfer to the isocyanate group. For that reason the first stage has been estimated as that limiting the general rate of the urethanisation reaction.