A series of Fe-Beta catalysts modified with different amount of Cu was prepared and tested in NOx selective catalytic reduction by NH3. ESR study reveals that replacement of isolated tetrahedral Fe3+ ions by Cu2+ ions in cationic sites of the zeolite takes place upon Fe-Beta treatment by copper. This modification results in gain in deNO(x) activity in low-temperature region, due to formation of cationic Cu2+, and significant loss in activity at high temperatures, which is determined by substitution of active Fe3+ for less active Cu2+. (C) 2010 Elsevier B.V. All rights reserved.