Mathematical modeling was used to simulate the dissolution of a single MnS inclusion within an electrochemical microcell. The model allowed for the evaluation of the hypothesis of mechanism of pit initiation based on a critical concentration of thiosulfate ions in the presence of chloride ions, which results in depassivation of the stainless steel. As a result of inclusion dissolution, the concentration of thiosulfate increased to values consistent with critical concentrations observed experimentally for pit initiation. Dissolution of inclusions led to modest local acidification, but not to the low pH values associated with passivity breakdown. The model defined the dimensionless parameter space under which thiosulfate and sulfide species would be expected to occur as a consequence of inclusion dissolution. Based on the hypothesis of critical thiosulfate initiation mechanism, the predictions of pitting potential for 304 stainless steel in chloride solutions were within experimental variation; the chloride concentration dependence was smaller than observed experimentally.