A new method was developed which fits a thermal internal energy distribution to ions formed by electrospray ionization. The molecular ion survival yield was measured and determined by RRKM calculations as a function of temperature. The (characteristic) temperature was determined when the calculated and measured molecular ion survival yields were equal. The characteristic temperatures were very similar (average RSD errors were 8%) for a set of analogous compounds (benzylpyridine salts), and the resulting thermal internal energy distributions were close to those determined by De Pauw's method. The validity of the method was also checked performin blackbody infrared radiation and on-resonance excitation experiments on a Fourier transform ion cyclotron resonance instrument with benzylpyridine salts and leucine enkephalin. The results strongly suggest that internal energy distributions in electrospray ionization are very close to thermal distributions. It was found that the characteristic temperature increases linearly with the cone voltage. It is suggested that the characteristic temperature can be used as a quantitative measure to control and standardize conditions in electrospray ionization.