Abstract The polarographic behavior of the hexaamminecobalt(III) ion was studied in solutions of polyphosphate with different numbers of chains. The decrease in the diffusion current, id with an increase in the phosphate (Cp)-to-hexaamminecobalt(III) (CCo) mole ratio was found to be proportional to 1/4 the power of the molecular weight, M, of the Co(NH3)63+-polyphosphate. This fact shows the associated ions between Co(NH3)63+ ions and polyphosphate anions are transported to the electrode by diffusion at a rate depending on the square root of the molecular weight of the diffusion depolarizer. From the shift of the half-wave potential in the presence of excess polyphosphate, the chemical compositions of the associated ion-pairs were determined as a function of log Cp/CCo.