Development of polyurea‐urethane and polyurea reaction injection molding (RIM) systems has created a need for kinetics of polyurea formation. Adiabatic batch reactions in solution were used to determine heats of reaction and relative reactivity of several aromatic amines and n‐butanol with phenyl isocyanate (PI). In addition to comparing times required to reach 25, 50 and 75% conversion for both catalyzed and uncatalyzed reactions, n‐th order models with Arrhenius rate constants were used to fit some of the exotherms. The reaction of 3,5‐diethyl toluene (2,4 and 2,6)‐diamine and PI could not be modeled due to unequal reactivity of the two amine groups. This unequal reactivity was studied using high performance liquid chromatography (HPLC) separation of the reaction products. The reactions of primary aliphatic amines and aromatic isocyanates were too rapid to be monitored in the batch apparatus. With a flow apparatus the reaction half time was estimated to be ∼ 0.002 s.