The stereochemical course of the phosphoric residue transfer from ADP to water catalyzed by the (Mg2+ + Ca2+)-dependent ATPase of sarcoplasmic reticulum has been determined. For this determination, the preparation is described of ATP gamma S, stereospecifically labeled in the gamma-position with both 17O and 18O. After hydrolysis of this nucleotide, the analysis of the product inorganic [16O,17O,18O]thiophosphate showed that the reaction proceeded with retention of configuration at the gamma-phosphorus atom. This result is expected since a phosphoenzyme is well characterized for this ATPase and provides support for the hypothesis that each phosphate transfer step occurs with inversion. In this case, the formation and breakdown of the phosphoenzyme occur each with inversion leading to the retention observed for the whole reaction.