The catalytic decomposition of hydrogen peroxide dissolved in heavy water, both on the surface of glass in alkaline medium and on metallic gold, was measured at 35 °C. The results, compared with those obtained under similar conditions for hydrogen peroxide in ordinary water, indicate that deuterium peroxide (D2O2) is much more stable than ordinary hydrogen peroxide towards such catalysts. The experimental results in both cases are of interest chiefly for comparison, and further investigations will be necessary before any sound conclusion can be drawn regarding the mechanism of the reactions involved.