A classical dielectric theory is developed to provide an expression for the energy loss of fast electrons passing through a metallic sphere at a fixed impact parameter, in the case of spheres coated by a thin oxide layer. The expressions for energy loss are valid for any dielectric function ε(ω). A high number of terms of the multipole expansion is required in order to take into account the effects of the oxide layer. Results are compared with experimental measurements of energy loss obtained by STEM techniques. © 1987 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division).