The present invention relates to a process for preparing hydrocyanic acid (HCN) by catalytic dehydration of gaseous formamide in a reactor which has an internal reactor surface made of a steel comprising iron and chromium and nickel, and also to a reactor for preparing hydrocyanic acid by catalytic dehydration of gaseous formamide, which reactor has an internal reactor surface made of a steel comprising iron and chromium and nickel, and also to the use of the reactor of the present invention in a process for preparing hydrocyanic acid by catalytic dehydration of gaseous formamide.