A systematic study of a variety of stainless steels [SS316, SS410, Jethete (J), Duplex (D), and Super Duplex (SD)] has been performed, measuring their electrochemical machining (ECM) characteristics in chloride and nitrate electrolytes. Theoretical current/time analysis using a segmented tool was used to determine the dissolution valencies, n, and overpotentials, V-0, along the flow path length. Electrolyte samples from the interelectrode gap, taken at intervals along the flow path, and the bulk were analyzed for conductivity, pH, and visible absorption. The results indicate that the ECM dissolution characteristics of stainless steels are controlled by the surface oxide structure, which is primarily determined by the chromium content. Typically, the high chromium steels D and SD were found to machine with high primary elemental valencies (consistent with n(Fe) = 3 and n(Cr) = 6), resulting in the production of soluble CrO4-2. However, for the low chromium steels J and SS410, low valency dissolution (consistent with n(Fe) = 2, n(Cr) = 3) occurred with chloride electrolyte, and this was also found for the intermediate chromium steel SS316 at low electrolyte flow and recycled electrolyte. These results have been explained by the disruption caused to the surface barrier oxide layer by chloride ions and solid ECM products, which allow low valency dissolution to take place. Significant accumulation of these products occurred when performing electrolyte recycling, which is commonly used in industry. (C) 2003 The Electrochemical Society.