The invention is directed to the preparation of organic or aqueous-organic hydrogen peroxide solutions by direct synthesis from a non-explosive gaseous mixture containing hydrogen and oxygen. The process is carried out in the presence of a noble metal catalyst, using a reaction medium containing a halide and a strong acid. It can be performed in a stainless steel reactor without corrosion occurring to the reactor material if, during the reaction, the surface of the stainless steel is, at no place, in permanent contact with the gaseous mixture passing through the reactor.