Vanadium-containing ZSM-5 zeolites were prepared by exchanging Na-ZSM-5 with vanadyl sulphate solution at different pH. The samples were investigated by spectroscopic techniques (UV-VIS, ESR), and their activity in the selective catalytic reduction (SCR) of NO by NH, was studied in a wide temperature range (450...870 K). At pH greater than or equal to 3, the exchange of vanadyl cations is accompanied by the deposition of small intra-zeolite vanadium oxide clusters, which can be avoided at pH less than or equal to 2,8. At elevated temperature (673 K), part of the vanadyl cations aggregates to V(V) clusters. Under more severe calcination conditions (823 K, air), these clusters can be disaggregated. Vanadyl-exchanged ZSM-5 catalysts are highly active in the SCR of NO by NH3 competing with V2O5/TiO2 catalysts. By comparing the catalytic behaviour of (VO)-ZSM-5 samples after specific pretreatments, isolated vanadyl cations could be identified as the active sites.