The location of both Mg and V ions in V, Mg-containing AIPO(4)-5 samples and the influence of the magnesium ions on the nature, location and redox properties of the vanadium species have been studied by combining several physico-chemical characterisation techniques (TPR, XPS, NMR and ESR). While homogeneously distributed V5+ ions in framework positions have been observed on calcined VAPO-5, extraframework V5+ species are mainly observed in calcined VMgAPO-5 sample. XPS and P-31 MAS NMR indicate isomorphous incorporation of magnesium ions in as-synthesised VMgAPO-5 sample. A partial extraction of magnesium ions from lattice position is observed after calcination. However, Mg2+ ions remain partially in framework positions as suggested by the presence of Bronsted acid sites in the calcined VMgAPO-5 sample. The presence of magnesium decreases the reducibility of V5+ ions, although the amount of reduced vanadium ions in VMgAPO-5 was higher than those in VAPO-5. A correlation between physico-chemical characteristics and catalytic behaviour is also discussed.