Abstract: A kinetic study is reported on the reaction between the complex trans-[CoCl2(en)2]Cl and the amino acids alanine and valine. Rate constants (k) were measured at several temperatures ranging from 25 to 75°C. Representative k (10-3 sec-1) values found for valine was 6.34 (25°C), 16.92 (35°C), 55.52 (45°C), 96.97 (55°C), 181.28 (65°C), 319.24 (75°C) and 7.33 (25°C), 20.09 (35°C), 53.82 (45°C), 122.03 (55°C), 214.03 (65°C), 343.13 (752°C) for alanine. The relative large variation in the rate constant as a function of the temperature can be related to the activation energy since, reactions that have lower Ea usually present fairly constant rates at different temperatures. The activation energy found for the substitution of alanine into the complex trans-[CoCl2(en)2]Cl was 66.6 KJ.mol-1 and for valine was 66.8 KJ.mol-1. Both reactions have presented mechanisms of the type SN1.