Process for preparing piperazine of the formula I by reacting diethanolamine (DEOA) of the formula II with ammonia in the presence of hydrogen and a supported, metal-containing catalyst has been found, wherein the catalytically active mass of the catalyst, prior to its reduction with hydrogen, comprises oxygen-containing compounds of aluminum, copper, nickel and cobalt and in the range from 0.2 to 5.0% by weight of oxygen-containing compounds of tin, calculated as SnO, and the reaction is carried out in the liquid phase at an absolute pressure in the range from 160 to 220 bar, a temperature in the range from 180 to 220° C., using ammonia in a molar ratio to DEOA used of from 5 to 25 and in the presence of 0.2 to 9.0% by weight of hydrogen, based on the total amount of DEOA used and ammonia.