In a method for producing hydrogen cyanide by passing a feed mixture comprising ammonia and methane through reaction tubes, coated on the inner surface with a catalyst comprising platinum, at a reaction temperature of 1000° C. to 1400° C., operated for a time period of at least 100 h, the concentration difference between the ammonia concentration and the methane concentration in the product gas mixture is maintained in a range of from 1.05 % by volume to 3.0% by volume for at least 80% of the time.