The heterogeneous fluorination reactions of CF CH Cl were performed over CrF supported on MgF -AlF and the 3 2 3 2 3 catalytic activities were compared with those of single support catalysts, CrF rMgF and CrF rAlF , in order to see any 3 2 3 3 enhancing effect of the mixed support on the catalytic activity. The catalytic activities of CrF rMgF -AlF catalysts were 3 2 3 considerably low compared to those of single support catalysts, CrF rMgF and CrF rAlF , indicating the existence of an 3 2 3 3 antisynergy between MgF and AlF . XRD measurements show that such an antisynergy effect is mostly attributed to the 2 3 formation of MgAlF P 1.5H O which seems ineffective as a supporting material. The observation of b-AlF in the XRD 5 2 3 Ε½ . patterns of the aluminum-rich AlrMg) 1 catalysts implies that MgAlF P 1.5H O or MgF might be responsible for the 5 2 2 transformation of g-AlF into b-AlF . FT-IR studies reveal that the fluorination activity is highly dependent on the strength 3 3 of Lewis and BrΓΈnsted acidities on the catalyst. The IR spectra of adsorbed CF CH Cl show that the intensities of 3 2 adsorption bands of CF CH Cl diminish as the amount of MgAlF P 1.5H O in the catalyst support increases. q 1998 3 2 5 2 Elsevier Science B.V. All rights reserved.