Samples of a commercial FCC catalyst impregnated with 3000 pprn vanadium and subjected to oxidative and reductive treatments, were characterized by electron spin resonance (ESR) and tested in a microactivity test (MAT) to study the effect of different oxidation states of vanadium on the catalytic behavior when processing an atmospheric residue from the North Sea. MAT testing at 524 degrees C shows that samples containing a substantial part of V4+ combined with a strong part of V3+ or lower oxidation states give a considerably less catalytic effect of vanadium than samples containing mainly V5+. The test results indicate that V4+ and V5+ have quite similar dehydrogenation activities, while V3+ or lower oxidation states have almost no dehydrogenation effect. It has been found that different oxidation states of vanadium (V3+ or lower, V4+ and V5+) co-exist when treating the catalyst samples in 5% hydrogen in nitrogen at elevated temperatures up to 760 degrees C. The initial conversion to V3+ or lower oxidation states occurs no later than after only one-third of the vanadium is converted from V5+ to V4+. A relatively higher concentration of V4+ is allowed when treating the catalyst in 5% carbon monoxide in nitrogen. (c) 2005 Elsevier B.V. All rights reserved.