Process for the reduction of a hexavalent chromium oxide based catalyst comprising contacting, under mixing conditions, a hexavalent chromium oxide based catalyst, with a molar excess, in respect of the chromium species, of a solid reducing agent and subjecting the mixture to a temperature in the range 30-1000.degree. C. The process is simple, involves low costs and allows to obtain a permanently reduced chromium catalyst that can be disposed without toxicity risks.