New silver-platinum bimetallic oxygen reduction reaction (ORR) catalysts are investigated in alkaline media both for kinetics and stability aspects. Polycrystalline materials and carbon-supported nanoparticles (Pt/C and Ag-Pt/C) are compared. All catalyst active areas are evaluated by hydrogen adsorption voltammetry and transmission electron microscopy. ORR pathway, studied on a rotating ring-disk electrode, involves four electrons per oxygen molecule both on bimetallic and pure platinum catalysts in classical sodium hydroxide solution (1 M NaOH at 25degreesC). In industrial medium (11.1 M NaOH at 80degreesC), only three electrons are exchanged on bulk platinum resulting from partial peroxide formation; these peroxide ions are chemically decomposed in the active layer for carbon-supported catalysts leading to an apparent four-electron pathway. ORR is shown to be first order toward oxygen both in classical and industrial sodium hydroxide solutions. ORR kinetic parameters for silver-platinum bimetallic catalysts are comparable to platinum, even in industrial medium. Carbon-supported catalysts aging behaviors in industrial medium demonstrate the highly instable nature of Pt/C both in open-circuit and polarization, leading to particle growth and consequently to active area decrease. Conversely, Ag-Pt/C remains stable under the same aging conditions. Ag-Pt/C, stable and active, is therefore a suitable catalyst for the air cathode in the membrane chlor-alkali electrolysis process. (C) 2003 The Electrochemical Society.