Abstract: The kinetics of cobalt extraction with P5709 has been studied in a Lewis cell. The aqueous phase consisted of Co2+ (0.5-5g/l) and Na2SO4 (9%) at pH = 5.5, and the organic phase, 15%(V/V) P5709 and 85%(V/V) kerosene, saturated with hydrogen. In order to determine the controlling regime of the extraction process, the stirring speed, the interface area between the two phases and the op-erating temperature were varied in the experiments,the results showed that cobalt extraction with P5709 was,mainly diffusion-controlled, with the following overall mass-transfer coefficients: Both experimental and calculation results showed that neither Kw nor Ko remained constant during extraction. The reasons for this variation wera discussed.