Abstract: The kinetic of reaction between isocyanates and alcohols is determined in a great extent by the presence in the reaction system of substances able to inhibit or to activating reaction rate of urethane formation. The most important effect is produced by compounds that appear as reaction products (urethanes, urea). By using a HPLC (High Pressure Liquid Chromatography) method to follow the evolution of reaction composition in usual condition of urethane synthesis, as like amine actions, by introduce certain amounts of amide or urea in the reaction system, this catalysed reaction rate of urethane formation. The same effect the urethane addition present also, but in this case the urethane hydrogen atom presence is necessary.