[en] Highlights: • N₂-adsorption on copper cluster cations was studied by mass spectrometry. • A pulsed magnetron sputtering in aggregation cell was used to form Cu_n⁺. • The N₂-adsorption reactivity of Cu_n⁺ decreased rapidly with n = 6–10. • A correlation between observed reactivity and calculated binding energy was found. We have observed gas-phase reactions between N₂ 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, N₂-adsorbed copper cluster cations, Cu_nN₂⁺, were observed in mass spectra. The N₂-adsorption reactivity of Cu_n⁺ was found to be relatively lower at n ≥ 10 than at the smaller n. The cluster-size dependence of N₂-adsorption on Cu_n⁺ has a correlation with binding energies between Cu_n⁺ and N₂ calculated by density functional theory.