By a new technique, experimental replacement of carbonates by sulfides is possible. A cylindrical core of marble with an axial cavity containing an oxidizing solution was suspended in a gold-plated bomb containing 5.5 molal NaHS solution (pH 8.6 at 25 degrees C) in equilibrium with nutrient sulfide minerals. The plastic lining of the cavity decomposed near run conditions of 400 degrees to 450 degrees C, 0.5 kilobar, to allow the oxidizing solution to penetrate the core from the inside simultaneously with immersion by the thermally expanding bisulfide solution. A narrow zone of replacement occurred within the core by reaction at the diffusion interface between the bisulfide and oxidizing solutions, simulating mixing of an ore solution with ground water.Results indicate that effective replacement of carbonate rocks proceeds from oxidation of ore-bearing bisulfide solution through interdependent reactions: (a) the oxidation reaction precipitates a film of sulfide along intergranular openings and may generate a small amount of acid in the presence of sufficient oxidizing agents (O 2 , N 2 , etc.) from circulating ground water, (b) the acid dissolves carbonates at the sulfide-carbonate interface to develop a narrow opening, (c) the new opening allows reaction of the bisulfide with partially depleted oxidizing solutions, thereby precipitating a new layer of sulfide but without dissolving more carbonate. Fluctuation in the relative rates of flow of bisulfide solution versus oxidizing ground water provides solution of carbonates when local conditions are more oxidizing and, when less so, of filling of open space by sulfides. These cycles, and replacement, continue as long as the supply of the two solutions is maintained intermittently.Bisulfide solutions under these conditions exhibit characteristics common to many mineralizing environments. Either quartz + calcite or wollastonite are compatible with bisulfide transport of sulfides, but quartz + calcite is favored by replacement reactions. However, quartz grains in marble are loci for sulfide deposition. Silica, barium, and most metals can be transported in bisulfide solutions and deposited simultaneously, as at Tintic, by interdependent reactions during oxidation, producing jasperoid, barite, sulfides, and replacement of carbonates.