The ionization and dissociation energies of the different uranium and plutonium oxides have been measured by mass spectrometry of molecular beams produced by Knudsen effusion at high temperature. The values obtained constitute a set of self-consistent quantities, which are in agreement with the existing thermodynamic data of these oxides. On the basis of the experimental molecular parameters, general formulas for the ionization and dissociation/ionization cross sections due to electron inelastic scattering have been obtained for collision energies up to about 60 eV. These formulas are sufficiently accurate to calculate the composition of equilibrium vapor mixtures over UO2 and PuO2 from conventional mass spectrometric measurements.