[en] α and eta, the ionization and attachment coefficients, and (E/N)*, the limiting breakdown electric field-to-gas density ratio, in SF₆ and SF₆ mixtures were calculated by numerically solving the Boltzmann equation for the electron energy distribution. The calculations require a knowledge of several electron collision cross sections. Published momentum transfer and ionization cross sections for SF₆ were used. Various attachment cross sections for SF₆ were measured by using electron beam techniques with mass spectrometric ion detection. A total cross section for electronic excitation of SF₆ was determined by comparing the predicted values of α, eta, and (E/N)* with measured values obtained from spatial current growth experiments in SF₆ in uniform fields over an extended range of E/N. With this self-consistent set of SF₆ cross sections, together with published He cross sections, it was then possible to predict the dielectric properties of SF₆--He mixtures. Published experimental values of α for these mixtures lie between the values of α calculated with and without ionization of SF₆ by excited He atoms. Published experimental values of (E/N)* agree with the calculations to within 5%. 11 figures