A process for preparing hydrogen peroxide which comprises reacting hydrogen with oxygen in an aqueous medium containing a platinum-group catalyst, characterized in that the partial pressure of hydrogen and the partial pressure of oxygen in the gaseous phase of the reaction system are maintained at at least 0.5 atmosphere and at least 1.0 atmosphere respectively, and the platinum group catalyst is caused to be present in an amount, calculated as metal, of at least a minimum effective catalytic amount up to 30 mg per 100 ml. of said aqueous medium.