We present momentum widths and mean kinetic energies of lithium and fluorine in 7LiF, as determined simultaneously from deep inelastic neutron scattering (DINS) measurements. Experimental data across a temperature range from 4 to 300K are presented, and these results compared to those calculated using a quasi-harmonic density-functional approach. In all cases, measured momentum widths are seen to be within approximately 5% of those calculated, despite the very low scattering cross sections of both 7Li and 19F. This is the first determination and comparison with theory of such simultaneous measurements for nuclei of mass >4amu, and demonstrates the implementation of the DINS method in its current form as a mass-selective neutron spectroscopy. © 2012 IOP Publishing Ltd.