The quantitative analysis of reflection electron energy loss spectroscopy (REELS) spectra of amorphous boron (a-B), amorphous carbon (a-C), and their respective nitrides (i.e., BNx and CNx) has been performed to obtain the energy loss function (ELF) and the electron inelastic mean free path (IMFP) of the respective materials. In the case of boron we have studied different thin films over a wide range of stoichiometries, i.e., B, BN0.4, BN0.6, BN0.8, BN0.9, and commercially available hexagonal boron nitride (h-BN). In the case of carbon films we have analyzed a-C, CN0.3, and CN0.5 since the nitrogen content could not be increased above CN0.5. The analysis of the respective REELS spectra has been performed according to the model developed by Yubero and Tougaard [Phys. Rev. B 46, 2486 (1992)] and Yubero et al. [Phys. Rev. B 52, 9719 (1996)]. In addition we have included the study of the influence of some of the input parameters (refraction index, density, gap energy, and momentum dispersion coefficient) required by the model, which, rather commonly, are not well known and are dependent on the composition. The ELFs determined in this study are compared with published data when available. The IMFPs are compared with values predicted by the TPP-2M formulas and values reported for similar compounds. (c) 2006 American Vacuum Society.