Abstract: The kinetics of cobalt extraction with extractant 5709 (Hexyl-phosphonicAcid Mono-1-Methylheptyl Ester )-Heptane-CoSO4-H2SO4 system was studied with a modified constant interface cell. The extraction kinetics was found to be a mixed controlling regime, i. e., diffusion of cobalt in aqueous phase and chemical reaction of the diffusion of 1 - 2 complex of cobalt at the interface. At higher pH, diffusion was the dominant step, at lower pH, the formation of complex was the dominant step. A mathematic model was established as follows: The apparent activation energy E thus calculated as 8.57 kcal/mol, also sup-ported the mixed controlling regime oi the extraction kinetics ot cobalt.