ZSM-5 zeolite was loaded with vanadyl ions (VO2+) by treatment of Na-ZSM-5 with aqueous VOSO4 solution at pH 1.5-2. The catalytic material was tested for the selective catalytic reduction of NO with ammonia at temperatures between 473 and 823 K and normal pressure using a feed of 1000 ppm NO, 1000 (or 1100) ppm NH3 and 2% O-2 in He. The catalyst proved to be highly active, providing, e.g. initial NO conversions of >90% at 6201 g(-1) h(-1) (approximate to 400 000 h(-1)) and 723 K, and selective, providing nitrogen yields equal to NO conversion at equimolar feed in a wide temperature range and only minor N2O formation at NH3 excess. Admixture of SO2 (200 ppm) resulted in an upward shift of the useful temperature range, but did not affect the catalytic behaviour at temperatures greater than or equal to 623 K. No SO2 conversion was noted at T less than or equal to 723 K and 4501 g(-1) h(-1). The poisoning effect of water (up to 4.5 vol%) was weak at temperatures between 623 and 773 K. VO-ZSM-5 catalysts are gradually deactivated already under dry conditions, probably by oxidation of the vanadyl ions into pentavalent V species. This deactivation is considerably accelerated in the presence of water. (C)1999 Elsevier Science B.V. All rights reserved.