The kinetics of complex formation between chromium(III) ion and ethylenediamine-N,N'-diacetic acid (EDDA) have been studied spectrophotometric ally. The reaction proceeds by a path which is first-order with respect to chromium(III) and EDDA. The rate increases with the increase of pH and temperature. The results are best accounted for by outer-sphere complexation equilibrium involving [Cr(H2O)(5)OH](2+) and EDDA. A rate equation is established which involves k and K-OS (the interchange rate constant and outer-sphere complexation). The hydrolysis constant of the complex was determined and the activation parameters for the reaction have been calculated.