Selective modification of surface and bulk V5+/V4+ ratios in Mo-V-Te-O catalysts has been achieved using nitric acid and saturated ammonia solutions as pH adjustors. The addition of nitric acid strongly suppressed V4+ in the bulk but encouraged it on the surface. Because the bulk-phase compositions of Mo-V-Te-O catalysts varied only slightly, the changes in the catalytic performance for selective propane oxidation to acrolein resulted mainly from the variation of the bulk and surface V5+/V4+ ratios. The high concentration of surface V5+ that was responsible for propane activation led to high selectivity of propylene and high conversion of propane. Increased bulk V4+ might result in oxygen vacancies, which promote the electron and oxygen transfer so as to increase the selectivity of deep oxidation products (COx). A low bulk V4+ concentration prefers selective oxidation products; the yield of acrolein was as high as 21.4%. (c) 2007 Elsevier Inc. All rights reserved.