The rate and mechanism of transfer of oxygen between thiosulfate ion and water, in the presence of sulfite, has been investigated with 18O-enriched Na2S2O3. The principal path, for the conditions examined, is via sulfur atom transfer between sulfite and thiosulfate, accompanied by oxygen atom transfer between sulfite and the solvent water. The latter path is suppressed at high pH, and oxygen transfer from thiosulfate proceeds to the sulfite stage only. At intermediate pH, the two processes have comparable rates, and the kinetics show the behavior expected for a system of consecutive, reversible first order processes. Under these conditions sulfite is observed to behave as a true intermediate, first accumulating 18O from thiosulfate, and then releasing it to the solvent water.