It is claimed that the poor agreement often found between the energy quantities measured by mass spectrometry, and those obtained by other methods, is due in large part to the interpretation of the experimental data. The conditions necessary for accurate measurement are summarized.It is shown that by making certain assumptions about the probability of ionization for different processes, ionization efficiency curves can be interpreted to give the probabilities for electronic transitions from the ground state to the various ionic states of molecules. From these electronic transition probabilities, information can be deduced about the potential energy functions for these upper states.To exploit the method fully, beams of ionizing particles with low spreads in energy are necessary. Electron and photon impact are compared, and it is shown that the latter possesses many advantages.