If small-angle scattering investigations are performed on aged Nimonic 80a, the signal consists of scattering contributions from carbides and γ' precipitates. The carbides account for a considerable amount of the scattering signal because of their high scattering contrast, although their volume fraction is much smaller than that of γ'. By combining small-angle X-ray and neutron scattering on the same samples, a simple model is developed to describe the scattering profiles. The model allows a separation of the contributions from carbides and γ' precipitates and shows good agreement with observations from transmission electron microscopy. By analysing the kinetics of γ' coarsening for different ageing temperatures, an estimate for the specific interfacial energy of γ' is obtained.