We define the interface excitation parameter (IEP) as the change in excitation probability, caused by the presence of a medium‐medium interface crossed by an electron, in comparison with an electron for which only bulk excitations are considered. This definition is established by analogy with the definition of the surface excitation parameter for which one of the two media of the interface is the vacuum and which has already been extensively studied. To calculate the IEP (as well as the energy‐differential IEP or DIEP), we generalize the model developed by Tung, Chen, Kwei and Chou [C. J. Tung, Y. F. Chen, C. M. Kwei and T. L. Chou, Phys. Rev. B 49 (1994) 16684] from dielectric response theory for surface excitation parameter determination. We perform these calculations for angles between 0o and 60o, for electron energies between 200 and 3000eV and for various combinations of materials, chosen for their academic (as Al/In) or practical interest (as SiO2/Si for instance). We show that for materials with “similar” dielectric properties (metal/metal), the IEP is completely negligible. On the contrary when the materials of the interface are characterized by a large energy band gap difference, as metal/insulator or semiconductor/insulator, the IEP can reach a value of about 0.26 for the smallest electron energies considered here. Moreover, we show that for the SiO2/Si interface, the energy‐differential IEP obtained from our model is in good agreement with previous experimental data. Copyright © 2015 John Wiley & Sons, Ltd.