Abstract: Attrition resistant agglomerates of active alumina, i.e., agglomerates displaying an attrition resistance of less than 0.3% after aging for 24 hours at 980.degree. C., are produced by establishing a batch of starting material active alumina agglomerates in a confining and protective treatment zone, such starting agglomerates having been impregnated with water or an aqueous acid solution and the confining treatment zone itself being enclosed within an autoclaving assembly as to provide an interspace between the internal wall surfaces of the autoclaving assembly and the exterior wall surfaces of the treatment zone, and hydrothermally treating such impregnated agglomerates at a temperature of at least 80.degree. C. by directing a stream of a hot heat transfer fluid against the exterior wall surfaces of the treatment zone.