Preliminary studies have been carried out on the rates of reaction of gaseous HCl with galena at temperatures ranging from 125 degrees to 532 degrees C. In the restricted temperature range 498 degrees -532 degrees C the reaction is of order 0.8 with respect to HCl and has a very small or zero value of activation energy. It is concluded that the reaction is predominantly heterogeneous in this region and that the rate determining step may be adsorption of HCl by the solid. At lower temperatures the heterogeneous reaction becomes complicated by retention of the relatively involatile product PbCl 2 by the surface. At significantly higher temperatures the reaction may become predominantly homogeneous.Rate studies on the conversion of sphalerite to ZnCl 2 by gaseous HCl indicate a zero order reaction (in respect to HCl) when HCl is present in large excess and temperatures in the range 550-620 degrees C are used. The activation energy for the reaction was found to be 35.8 Kcal per mole of ZnS converted. Comparison with the rate of attack of HCl on galena (PbS) and Chalcocite (Cu 2 S) gave an order of PbS> or =ZnS> or =Cu 2 S.The results of these rate studies are used to explain zoning of copper and zinc which is observed in the Noranda type deposits of Canada.