The role of [Cu II (NH 3) 4] 2+ complexes in aqueous solution has been investigated for the catalytic reduction of nitric oxide with ammonia, and these results have been compared with those reported previously in which a Y-type zeolite was employed as a support. In aqueous solution the reaction was found to go through a reversible rate maximum at 55 °C which is about 55 °C less than the temperature of the maximum in the zeolite. The decrease in activity at high temperatures was accompanied by a decrease in Cu(II) concentration. The reaction rate was second order in nitric oxide compared with first order in the zeolite. The addition of nitrite and nitrate ions to the aqueous catalytic solution yielded no observable effect on either the reaction rate or product distribution. A reaction mechanism has been proposed in which the rate limiting step at temperatures less than 55 °C is the reaction of N 2O 2 with coordinated NH 3. At temperatures above 55 °C, the reoxidation of Cu(I) back to the Cu(II) complex becomes the slow step. This study further demonstrates the marked differences which can occur when the same reaction is catalyzed by a transition metal complex in two different media.