We measured solubilities of nitrogen and argon in basalt melt at 1300 °C under highly oxidizing conditions (P(O2)=0.16‐0.37 atm) using 15N15N‐labeled air (Ptotal=1 atm). The obtained solubilities (Henry’s constants) are K(N2)=(1.3‐2.9)×10−9 mol/g/atm and K(Ar)=(2.6‐4.5)×10−9 mol/g/atm, respectively, showing that the solubility of nitrogen is slightly lower than that of Ar under highly oxidizing conditions. The 15N15N‐labeled gas used in the solubility experiment was not isotopically in equilibrium but highly enriched in 15N15N, that is, the ratios of 14N14N: 14N15N: 15N15N in the gas were not 1: 2r: r2, where r=15Ntotal/14Ntotal. The recovered gas from the synthetic basalt glass also showed similar isotopic disequilibrium. This suggests that isotopic exchange among N2 molecules did not occur during dissolution of the gas in the melt. In other words, nitrogen dissolved in the basalt melt as molecules in the present (highly oxidizing) experimental conditions.