To prepare 2, 4, 10 and 15% Co catalysts, gamma-Al2O3, ZrO2 and SiO2 were impregnated with aqueous Co(NO3)(2).6H(2)O for 24 h at room temp., H2O was evaporated, the residue was dried 4 h at 105degreesC and calcined 3 h at 350degreesC in air stream. The catalysts, 0.2 g each, were reoxidized at 400-800degreesC in 50 mL/min (95:5 v/v) O-2-He, reduced at T raised linearly 20degreesC/min up to 1000degreesC in 50 mL/min (95:5 v/v) H-2-Ar, and cooled in a neutral gas. Diffractograms were recorded with Cu-K-alpha (156.06 pm) for catalysts oxidized or reduced 2 h at 400(alpha), 600, 800 or 1000degreesC. In CoO/Al2O3 systems (Fig. 1), the species reduced included: (i) over 200-300degreesC, small CoO and Co3O4 crystallites (not interacting with carrier) deposited on large Co aluminate crystallites; (ii) at about 500degreesC. topical CoO (highly dispersed and partly interacting with carrier), vanishing in catalysts reoxidized at 800degreesC; (iii) at 750degreesC, nonstoichiometric aluminates CoxAl8/3x (0900degreesC, carrier-incorporated Co2AlO4, and CoAl2O4. The CoAl2O3 (carrier) interactions were strongest; in CoSiO interactions were strong only when the catalyst was oxidized at high temperatures; in Co-ZrO2, no interaction occurred regardless of Zr content and reoxidation temperature.