The problem of strong space-dependent neutron spectra appearing at boundaries is studied at a sodium-iron interface in slab geometry. The spatial neutron spectra from a 14-MeV neutron source were measured in the energy range from 100 eV to 4.5 MeV. For the energy E < 200 keV, the time-of-flight method was employed and for E > 200 keV, proton-recoil detectors were used. In addition, calculations were performed with the Karlsruhe transport codes DTK and SNOW. The comparison of measured and calculated spectra, both functions of space and energy, showed that the spectrum around the sodium resonance is well described by 208 energy groups. A relationship is developed that is a function of the resonance parameters and the properties of the material concerned, from which it is possible to estimate the depth to which the flux depression at a resonance energy propagates into an adjacent material.