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Information on interfacial mass transport of cadmium in cadmium sulfide suspensions and er equilibrium conditions has been obtained by probing with a radiotracer for cadmium. The transport of Cd2+ ions from the cadmium sulfide solid particles to the saturated solution and visa versa is followed by adding radioactive 109Cd2+ to the solution and measuring the amount of radioactivity present in the solution as a function of time. The amount of exchangeable cadmium in or at the solid/liquid interface, which is deduced from the final value of radioactivity in the solution appeared to be 1.8 to 2.3 times the amount of cadmium in one lattice layer. Compartmental analysis of the experimental data revealed that 4 different cadmium species are involved in the exchange processes, one of which being the cadmium in the bulk of the solution, and another one, representing 40% to 50 of one lattice layer, which exchanges rapidly with the solution in comparison with the other two species left.