The distribution of vanadium oxidation states in a 5.9 wt% V/ Al2O3 catalyst reduced in H-2 or CO was determined from XPS V 2p spectra, Principal component analysis (PCA) was used to statistically determine the number of components (oxidation states) that describe the V 2P(3/2) envelope of the reduced V/Al2O3 catalyst. The peak positions and FWHMs of the components were obtained from iterative target transformation factor analysis (ITTFA). The V 2P(3/2) envelope is composed of two components (oxidation states), which are centered at 517.2 and 515.5 eV (FWHMs of 2.6 and 2.3 eV, respectively) when H-2 is the reducing agent and at 517.3 and 515.8 eV (FWHMs 2.7-2.8 eV) when CO is the reducing agent. The components were assigned to V5+ and V3+ based on the difference in the binding energies and the results of a volumetric study of catalyst reduction in CO. The spectral information gained from PCA and ITTFA was used to curve fit the V 2p envelopes. The extent of reduction of the V phase was estimated from the relative abundances of V5+ and V3+ determined by curve fitting and corrected for the presumably lower dispersion of the V3+ species. The results indicated a decrease in the average oxidation state of V to 3.5 with increasing the reduction temperature in H-2 up to 662 degrees C, Reduction in the presence of CO was measured volumetrically and showed a decrease in the average oxidation state from 5 to 3.3 over the temperature range 155-510 degrees C. Curve fitting the spectra of the GO-reduced samples using the two spectral components identified through factor analysis yielded an average oxidation state of ca. 3.6 upon reduction at 510 degrees C.