A rotating ring hemispherical electrode was used to study the active dissolution of mild steel in sulfate electrolytes. Using potentiostatic transients for the deposition of a monolayer of an iron oxide on a platinum ring electrode, the soluble FeOH+ complex ion was identified as an intermediate product during the dissolution reaction. The FeOH+ concentration was found to increase with pH of the electrolyte and to decrease with sulfate concentration. In concentrated sulfate solutions, the FeOH+ concentration also decreased with increasing dissolution current density at the mild steel hemispherical electrode. The results indicate that the dissolution reaction did not fully follow the commonly accepted Bockris or Heusler mechanism. This discrepancy is discussed with a parallel mechanism, in which sulfate ion participates in the anodic reaction.