Electrochemical impedance diagrams have been measured under controlled mass‐transport conditions for the transpassive anodic dissolution of , a stainless‐steel type alloy, in concentrated phosphoric‐sulfuric acid electrolytes under both polishing and nonpolishing conditions. The impedance diagrams consist of four parts, each corresponding to a particular frequency range. At the highest frequencies the impedance results from a charge‐transfer process, and at the lowest frequencies from a diffusion process for water or a water‐related species which may be acting as an acceptor for dissolved metal ions. Changes in the shape of the impedance diagrams with temperature and water concentration are particularly visible in the lowest frequency range. The brightness of a polished surface depends on the importance of the diffusion process in the overall mechanism for metal dissolution, and it varies continuously as a function of temperature, potential, and water concentration. The results contain no evidence suggesting the presence either of a porous film behaving as an ohmic resistance or of a compact film through which ions migrate by high‐field conduction.