The heats of reaction of normal-, iso-, and secondary-butyl alcohols reacting with phenyl isocyanate, the three tolyl isocyanates, and 2,4-tolylene diisocyanate were measured at 25 °C, using a differential calorimeter of the Tian–Calvet type. The results are interpreted in terms of substituent effects.From bond energy considerations the heats of formation of phenyl isocyanate and tolyl isocyanate have been estimated.