The air-calcined chromia-based catalysts and nitrogen-calcined chromia-based catalysts are prepared and used for a vapor-phase catalytic fluorination reaction in preparing difluoromethane. It has been found that the air-calcined catalysts show much higher catalytic activity than that of nitrogen-calcined catalysts in the above mentioned reaction, which is attributed to the formation of a Cr species with high oxidation state on the air-calcined catalyst. Moreover, when the catalyst precursor is treated with anhydrous hydrogen fluoride at low temperature, the crystallized catalyst is easily formed and the catalytic reaction activity is decreased. (C) 2004 Elsevier B.V. All rights reserved.