Pt–Pd aggregated nanoparticles were immobilized on a Nafion-117 membrane to decompose hydrogen peroxide. The Pt or Pd particles alone decompose hydrogen peroxide at an insignificant rate. But a bimetallic Pt–Pd catalyst, having a Pt to Pd composition between 0.33 and 0.75, causes steady decomposition at an appreciable rate at room temperature. The Pd and Pt sites have been proposed to initiate and to complete, respectively, the impulsive hydrogen peroxide decomposition reaction.