Aluminum-doped copper cluster cations, CunAl+, were produced via an ion sputtering method and analyzed by mass spectrometry. The measured size distributions show that Cu6Al+ and Cu18Al+ are highly stable species, which can be understood in terms of the electronic subshell 1P and 2S closings, respectively. Furthermore, the reactions of size-selected CunAl (n = 4-6 and 8-16) with NO and O-2 were studied at near thermal energies by using a tandem-type mass spectrometer. The doping of an Al atom improves the reactivity of the clusters toward NO in particular for n = 9, 11, 13, and 15, whereas it does not change the reactivity toward O-2 significantly. Consequently, it was found that CunAl+ (n = 9, 11, 13 and 1.5) are more reactive toward NO than toward O-2. The high reactivity of Cu9Al toward NO compared to that of Cu-10(+) is explained in terms of the increase of the adsorption energy and the lowering of the barrier to dissociative adsorption, with the aid of calculations based on density functional theory. Moreover, the multiplecollision reactions of Cu,Al+ (n = 9, 11, and 13) with NO result in the production of cluster dioxides, Cun-3AlO2, (i.e., release of N-2), which clearly indicates that NO decomposition proceeds on these clusters.