The structure and morphology of thin film catalysts, consisting of well-defined Rh particles in contact with promoting and supporting vanadium oxides and subjected to oxidative and reductive treatments at elevated temperature, were correlated with the associated activity changes in CO hydrogenation. All systems exhibit a steady decrease of the catalytic activity with reduction temperature (373-823 K), which is in part due to vanadia covering free metal surface area and in part to the formation of stable bulk alloy phases. By comparison with the results of electron microscopy, the initial decrease of catalytic activity (between 373 and 473 K) is primarily caused by decoration of Rh particles by reduced VOx species. A strong decrease of catalytic activity after reduction above 573 K is related the formation of distinct Rh-V alloys. A selectivity shift towards methane at high reduction temperatures goes along with this activity loss. (c) 2006 Elsevier B.V. All rights reserved.