A study was made of the precipitation, by means of zinc dust, of copper and cadmium from a "normal thickener overflow" solution as obtained in the hydro-metallurgy of zinc. The effect of (i) the quantity of zinc dust added, (ii) the initial pH of the solution, (iii) the time of agitation, and (iv) the aeration of the solution were studied. The polarograph was used to follow the changes in the concentration of the solution.For the effective removal of cadmium, the initial pH of the solution should not be below 3.5. The time of treatment with zinc dust should be controlled within narrow limits and, while the agitation should be effective, it should not cause aeration of the solution. The redissolving of cadmium on prolonged treatment appears to be a typical process of electrolytic corrosion analogous to underwater corrosion of iron in the neutral zone, the rate of redissolving being dependent on the supply of oxygen for depolarization. Similar redissolving of copper does not take place under the conditions studied and, given sufficient zinc dust, only the minimum time of treatment need be controlled.