AbstractThe temperature‐jump method has been used to determine the nickel(II)‐ and cobalt(II)‐arginine complexation kinetics. In the pH range studied, the neutral form of the ligand, HL, is the attacking, as well as the complexed, ligand species. The reactions reported on are of the typewheren= 1, 2, 3 and M is Ni or Co. At 25° and ionic strength 0.1Mthe association rate constants are: for nickel(II)k1= 2.3 × 103(±20%),k2= 2.4 × 104(±20%),k3= 3.5 × 104(±40%)M−1sec−1; for cobalt(II)k1= 1.5 × 105(±20%),k2= 8.7 × 105(±20%),k3= 2.0 × 105(±40%)M−1sec−1. Arginine binds to metal ions less well than homologous chelating agents due to the electrostatic repulsion arising from the positively charged terminus of the zwitterion. Kinetically, the effect appears in the association rate constants with nickel reactions more stron