The speciation of vanadium in initial aqueous NH4VO3 solutions as a function of pH and concentration, in supernatant and in wet solids were investigated by 51V static and MAS NMR. Two series of VSiBeta zeolite catalysts were prepared by a two-step postsynthesis procedure at pH = 2.5 and 6. 51V static and MAS NMR, 51V 3Q MAS NMR, DR UV–vis, XPS and EPR allowed determining the state of vanadium in both series of catalysts. The catalytic activity of VSiBeta catalysts in SCR of NO strongly depended on the state of the vanadium present. The V-single site V1.0SiBeta(I) and V1.4SiBeta(I) catalysts with isolated pseudo-tetrahedral V(V) were active in SCR of NO with NH3 process, with maximum NO conversion about 75 % at 773 K for V1.4SiBeta(I). In contrast, V1.0SiBeta(II) and V7.5SiBeta(I) catalysts containing pseudo-octahedral V(V) were much less active in SCR of NO and high amount of undesired N2O was produced.