This report describes the results of flow tests on the rate and ultimate yield of uranium and pyrite leaching under conditions representative of carbonate in situ uranium leaching. The effects of cation type, ph, carbonate and oxygen concentrations, and flow rate were investigated for uranium leaching. Leachant cation, ph, and carbonate concentration had little effect on uranium leaching, but the leaching rate depended on oxidant concentration to 1/2 order. The insensitivity to cation type led to a new process whereby the leach solution is preceded by a pre-flush containing the chloride of the leaching cation. The preflush prevented permeability losses assocciated with carbonate precipitation and clay swelling caused by certain cations. The effects of leachant cation, ph, carbonate and oxidant concentration, and particle size were also studied for pyrite leaching. The reaction products were elemental sulfur, thiosulfate, sulfate, and an unidentifiable sulfur species. The fate of iron was geothite, amorphous iron, oxyhydroxides, and lepidocrocite.