In support of its goal to maintain an adequate supply of minerals to meet national economic and strategic needs, the Bureau of Mines has investigated a novel hydrometallurgical system with sulfur dioxide. Differential leaching with dissolved so2 of manganese, nickel, and cobalt from their hydrous oxides was found to be very rapid and efficient--almost instantaneous and quantitative. In this method a given weight of mixed oxides previously ground to minus 200-mesh is slurried with a leach solution containing a specified quantity of dissolved so2 for 10 minutes at room temperature and ambient pressure. An empirical leaching parameter, r, is defined as the ratio of the number of so2 moles in the leaching solution to the weight of the metal oxide charge. Plots of the variation of metal extraction with the ratio r generate a series of sigmoidal-shaped curves, each of which is characteristic of a specific metal extraction. A threshold value of r is required to initiate the leaching of a given metal from the mixed oxides. Once this threshold is reached, the metal recovery can increase to more than 95 pct in very short times.