Abstract A new experimental set-up using an auxiliary electron gun allows to record highly resolved electron energy loss (HREEL) spectra on insulating samples. Surface potential stabilization is interpreted in terms of induced surface conductivity and enhanced secondary emission. Results are presented for MgO(001), Al 2O 3(0001),SiO 2(0001), LiF(001) and NaCl(001). Infrared optical constants can be retrieved from the HREEL spectra by data processing with the Dielectric Theory. These constants are obtained as accurately and independently on infrared Spectroscopy. Other examples of applications are given for non single crystalline insulating materials.