The rates of reaction of glycerol and two polyether macrodiols, differing in molecular weight by a factor of 6, with a commercial grade of liquid polymeric 4,4’-diphenylmethane diisocyanate (MDI) were determined using data from reaction calorimetric studies. Rate constants were determined for the reactions of each of the macrodiols and glycerol with the isocyanate. The reactivities of the primary hydroxyl groups in glycerol were similar to those of the macrodiols. The more sterically hindered secondary hydroxyl group in glycerol was found to have a reactivity one third that of the primary hydroxyl groups.