Adsorption of CO on VAPO-5 and VMgAPO-5 has been studied by IR spectroscopy. No carbonyl species were formed with V5+ sites on oxidized VAPO-5, Mild reduction (673 K) of the sample with hydrogen generated V4+ sites which did not form stable complexes with CO at ambient temperature. At 85 K, however, two V4+-CO species were detected, characterized by bands at 2200 (A species) and 2194 cm(-1) (B species). In these complexes CO is bonded via a sigma-bond only, Deeper reduction (773 K and above) led to the formation of V3+ sites, predominantly at the expense of the A species. These cations formed two kinds of carbonyl complexes (bands at 2197 and 2185 cm(-1)) in which a weak pi-back bonding is realized. As a result, the carbonyls were more stable than the V4+-CO species and could be detected even at room temperature. The V3+ sites were fully oxidized by oxygen even at 85 K thus forming V4+ and V5+ species. At higher reoxidation temperatures (up to 373 K) the major part of the V4+ sites was also oxidized to V5+, CO adsorption on VMgAPO-5 results in the formation of Mg2+-CO species producing intense bands at 2187 and 2179 cm(-1) which were superimposed on the Vn+-CO bands. For this reason the oxidation-reduction behavior of vanadium on this sample could not be studied in detail. It was found that some V4+ sites (producing a band at 2204 cm(-1) at low temperature CO adsorption) were generated during the evacuation at 673 K, Deeper reduction with hydrogen created new sites (most probably V3+) which were characterized by a carbonyl band at 2197 cm(-1). The nature of the Vn+-CO bonds as well as the effect of different factors on its strength are discussed. Some conclusions on the reduction-oxidation behavior of vanadium in the VAPO-5 and VMgAPO-5 are also presented.