A hydrocarbon hydroconversion catalyst is regenerated by (a) combustion, (b) chlorination or oxychlorination and (c) treatment with an oxygen-containing gas, and the gas discharged from the regeneration zone, before re-use, is (a) cooled down to below 70.degree. C., (b) subjected to double washing and (c) dried to reduce the water content of the gas to less than 2,500 ppm. An apparatus is also disclosed, certain parts of which are made of a material strongly resistant chlorine and other parts of a less resistant material with respect thereto.