It is of paramount importance to know the total elastic-scattering cross sections of the gases which are present in the specimen chamber of the variable pressure scanning electron microscope (VP-SEM) or in the environmental SEM (ESEM) since these values are needed to compute the width of the beam broadening skirt by the use of Monte Carlo simulations. However, these values are not well known for even the most common gases because the vibrational modes of the molecules add another contribution to the usual value of the elastic scattering cross-section. To overcome this difficulty a method has been proposed to measure total elastic cross-sections which is based on a measurement on the unscattered beam current as a function of the chamber pressure using a Faraday cup. The total elastic cross-sections are then deduced using the well-known exponential decay law.