Reactions of oxygen-chemisorbed cobalt and iron cluster cations (ConOm+ and FenOm+; n = 3-6, m = 1-3) with an NH3 molecule have been investigated in comparison with their bare metal cluster cations at a collision energy of 0.2 eV by use of a guided ion beam tandem mass spectrometer. We have observed three kinds of reaction products, which come from NH3 chemisorption with and without release of a metal atom from the cluster and dehydrogenation of the chemisorbed NH3. Reaction cross sections and branching fractions are strongly influenced by the number of oxygen atoms introduced onto the metal clusters. Oxygen-chemisorbed metal clusters with particular compositions such as Co4O+, Co5O2+, and Fe5O2+ are extremely reactive for NH3 dehydrogenation, whereas Co4O2+ and Fe4O2+ exhibit high reactivity for NH3 chemisorption with metal release. The enhancement of dehydrogenation for specific compositions can be interpreted in terms of competition between O-H and neighboring Co-H (or Fe-H) formation.