A new method for the determination of association constants by the measurement of chronopotentiometric transition time has been developed and applied to the study on the outer-sphere association of some substitution-inert complex cations with sulphate ions. The association constants were determined from the change in transition time as function of the concentration of sulphate ions at ionic strength 0.1 (NaClO4) and 25° C: 93 ±8 for [Co(NH3)6]3+SO42−, 99 ±15 for [Co(en)3]3+SO42−, 61 ±7 for [Cr(NH3)6]3+SO42− and 58 ±8 for [Cr(en)3]3+SO42−. The ratios of the diffusion coefficients of the univalent ion-pairs and tervalent substitution-inert complex cations were also obtained. From these ratios the diffusion coefficients of the univalent ion-pairs were calculated, which were found to differ not greatly from those of univalent complex ions with similar structures.