When chromia is employed as a catalyst for heterogeneous chlorine/fluorine exchange reactions drastic modifications in the solid surface regions occur due to chemical reactions with the gas phase. As a result the solid develops its full catalytic activity. Certain amounts of halides (chloride as well as fluoride) are analytically detectable but XRD does not reveal any changes due to small amounts and/or the amorphous character of the surface products, XPS and XAES have been employed to obtain further information on the phases formed during the conditioning process. Different chromium compounds (oxides and fluorides) have been investigated as reference samples. The catalytic behavior has been determined and compared with changes of the photoelectron and Auger electron spectra of the solid samples, From the results it can be stated that a catalytically active phase is formed in all cases during the conditioning process. This phase exhibits chemical and topological properties very similar to beta-CrF3. By contrast, alpha-CrF3 is completely inactive catalytically, Explanations are given on the basis of structural differences between the two CrF3 modifications.