The acid elution of the dark blue cupric ammine complex from a cation-exchange resin produced a sharply defined moving boundary within each bead, which was photographed and measured to provide rate data. 'Analysis of the data by means of a theoretical model indicated that the overall elution process was controlled by a combination of internal and external mass transfer when acid concentrations less than 1.0 N were employed. At these acid concent1'ations the interdiffusion coefficient for the resin phase was found to be 3 X 10-6 cm2/sec, independent of acid concentration but somewhat dependent on particle size. Data collected at· higher acid concentrations were not amenable to analysis by the method employed. The model should be useful for predicting the time required to elute the cupric ammine complex or for designing equipment to carry out this process.