A reactor for preparing hydrogen cyanide by the Andrussow process is provided. The reactor comprises at least one gas inlet which opens into a gas inlet region, an outlet for the reaction products and a catalyst, wherein at least one mixing element and at least one gas-permeable intermediate layer are within the reactor between the gas inlet region and the catalyst. The mixing element is arranged between the gas inlet region and the gas-permeable intermediate layer. A process for preparing HCN, in the reactor is also provided.