The catalytic activity of the air-calcined catalyst of CrF3 . 4H(2)O for the fluorination reaction of CF3CH2Cl was found to be 20 times higher than that of CrF3 . 4H(2)O. Photoacoustic spectroscopy measurements and XPS revealed the existence of various chromium phases and an increased density-of-states near the Fermi energy level in this catalyst. The increased catalytic activity of the air-calcined sample is attributed to the relatively easier formation of labile fluorine on the catalyst surface due to the stronger interaction between the valence band of CrOxFy and the antibonding orbitals of HF. (C) 1998 Academic Press.