The breakdown voltages of some electron attaching gases and gas mixtures have been calculated from their fundmental properties such as ionization and attachment cross sections, the momentum transfer cross section, the drift velocity or the mean energy of electrons. The calculated breakdown voltages in SF6, N2O, SF6+N2, and SF6+N2O gas mixtures, on the basis of Maxwellian electron energy distribution, agree very well with the measured values. The good agreement between the calculated and measured values shows that the mean energies of electrons in SF6+N2corresponds to that of SF6up to 90% concentration of N2in the mixture, whereas the mean energies of electrons in SF6+N2O mixtures correspond to that of the majority gas.