A static equilibration method was used to measure the single stage separation factor α of isotopic pairs in the vapour liquid equilibrium of A, N2 and O2 below boiling-point. Since the corresponding isotopic mixtures are supposed to be ideal, the vapour pressure ratio of the studied isotopic liquids is identical to α. Due to the sensitivity and the precision of the mass spectrometer employed, enriched isotopes needed not be used. No high purity of the gases was necessary since, at least to a certain extent, the adopted method of measurement is insensitive to impurities. The separation factors at the boiling point were found to be the following:36A-40A α=1.0060±0.0001,16O2-16O18O α=1.0050±0.0001,14N2-14N15N α=1.0037±0.0001. The temperature dependence was established in all cases in a relatively narrow range of temperatures. A discussion of the argon results in the light of the quantum theorem of corresponding states is given.