A series of iron-exchanged zeolite beta (BEA) has been characterized by temperature programmed desorption (TPD) of oxygen and tested in the catalytic reduction of N2O by NH3. TPD of O-2 has clearly shown that iron-oxo species formed upon reoxidation by nitrous oxide of Fe2+-BEA are more labile than those generated by calcination in ain In the selective catalytic reduction of N2O by NH3 the conversion of N2O increases until a degree of iron exchange level of 100% and then decreases. In contrast, the intrinsic activity per mole of iron decreases drastically with the increase of iron content. Mononuclear iron-oxo species have been proposed as the most active sites for the reduction of N2O by NH3; binuclear iron-oxo species and iron oxides aggregates are less active.