Separations by thermal diffusion in the mixtures 14N15N/14N2 and 15N2/14N2 have been made at average temperatures between 152°K and 470°K. The results are consistent with the CHAPMAN-ENSKOG theory in sharp contrast to separations in isotopic CO mixtures 1. From the difference in behaviour between CO and N2 it is concluded that energy transfer between translation and rotation of the molecules is not responsible for the influence of isotopic substitution on the thermal diffusion factor.