Chromium oxides dispersed on various supports by precipitation method were tested for the catalytic fluorination of HCFC-133a to HFC-134a. Catalytic activity decreased in the order of MgO>Al2O3>MgF2>TiO2>ZrO2. The most active Cr/MgO, when properly activated and pn-fluorinated before the reaction, shows superior activity to other catalysts that have been reported so far. Isolated (monomeric) Cr(VI) species that was formed during an optimized thermal activation were changed into fluoride (CrxFy.) and oxyfluoride (CrxOyFz) form of monomeric and oligomeric Cr species during the pre-fluorination with HF, the latter of which are believed to be active sites for the reaction. High pre-fluorination temperature accelerates the formation of crystalline Cr2O3 due to the formation of crystalline MgF2 from MgO which anchors the isolated Cr(VI).