We have observed gas-phase reactions between N-2 molecules and copper cluster cations, Cu-n(+) (n = 2-17), formed by a modulated pulsed power magnetron sputtering cluster ion source. By introducing N2 gas effusively between the source and an acceleration region of a time-of-flight mass spectrometer in the vacuum chamber, N-2-adsorbed copper cluster cations, CunN2+, were observed in mass spectra. The N-2-adsorption reactivity of Cu-n(+) was found to be relatively lower at n >= 10 than at the smaller n. The cluster-size dependence of N-2-adsorption on Cun+ has a correlation with binding energies between Cu-n(+) and N-2 calculated by density functional theory. (C) 2017 Elsevier B.V. All rights reserved.