Measurements of the slow neutron scattering cross-section at 0·5 Å for nitrogen gas and oxygen gas at room temperature and low density are reported. The results are interpreted in terms of a new quantity, the structural second virial coefficient,B2(Q, T), whereB2(O, T) is the conventional second virial coefficient. It is shown that for nitrogen the observed function, or the ‘difference’ cross-section, corresponds quite well to that calculated for an isotropic Lennard-Jones potential and less well for recently proposed anisotropic potentials. The method is promising for the determination of interaction potentials for molecules.