A detailed kinetic study of the reaction of aquacobalamin [H2OCbl(III)] with hydrogen sulfide in water at pH 1 to 10 was carried out using conventional spectrophotometric, stopped-flow, and high-pressure stopped-flow techniques. The reaction proceeds in three steps: formation of the complex between aquacobalamin and hydrogen sulfide, inner-sphere electron transfer, and addition of a second molecule of hydrogen sulfide to the reduced cobalamin. Kinetic parameters for the different reaction steps of the studied process were determined. The final products of the reaction between aquacobalamin and hydrogen sulfide were found to depend on pH. The main reaction product is five-coordinate base-on reduced cobalamin(II) [B12r, Cbl(II)] in alkaline medium and six-coordinate cobalamin(II) with the anion-radical SSH2– in acidic medium.