Abstract High resolution electron energy loss spectroscopy applied to study the surface (Fuchs-Kliewer) phonons of cleaved (100)LiF and (100)NaCl reveals one peak respectively at h ̵ ω = 71 meV and 29 meV . These values are in agreement with the theoretical ones deduced from the optical constants determined by IR spectroscopy. It is demonstrated that the IR optical constants can be obtained from the HREELS data fitted to the Dielectric Theory. The high resolution of the spectra in the energy loss and gain regimes allows a fine structure in the broad peaks of LiF to be observed. This is interpreted by a subsidiary oscillator contribution whose origin is discussed.