Reactions of goethite and magnetite with sulfide solutions under CH4 and/or CO2 atmospheres were monitored in situ with Raman spectroscopy in the high-pressure capillary optical cells (HPOCs) at room temperature. Isolated systems were created in the enclosed HPOCs for the study of the mineral–solution interactions to prevent oxidation during the experiments. We observed that iron (oxyhydr)oxide minerals were inert to sulfide solution under CH4 atmosphere, and that the addition of CO2 to this system triggered the sulfidization reactions and produced iron monosulfide; mackinawite was identified in the solid phase immersed in the solution after six hours and pyrrhotite in the solid phase exposed to the vapor phase after 5 days. Mackinawite formed in the aqueous phase was metastable, and dissolved after about 10 days. The observed sulfidization of iron (oxyhydr)oxide minerals was through a replacement reaction rather than a dissolution–precipitation process.