The effect of internal vibrational-rotational thermal energy on the electron-induced fragmentation observed in mass spectra was studied both experimentally and theoretically. The combination of effusive molecular beam and a fly-through quadrupole mass spectrometer was used to study this effect in 1-iodopropane and 2-iodopropane. Although the temperature effect on the fragmentation is more pronounced at low electron energy, it was found to be substantial even at 70-eV electron energy on the molecular ion. The relative decrease of the molecular parent ion with the neutral parent vibrational temperature followed an Arrhenius-like temperature dependance with an observed "activation energy" which was largely electron energy independent. The fragmentation patterns were simulated by using the maximum entropy formalism including all possible fragmentation pathways. The theoretical calculations compare well with the experimental results giving credence to the main approximation used of fully statistical mass spectral fragmentation pattern. It follows that the mass spectrum is governed primarily by the mean energy content of the parent ion. The observed internal energy effect can therefore be used as a thermometer in the study of translational-internal energy transfer in a hyperthermal 1-iodopropane-diamond scattering and the separation of this energy from the energy transferred to the surface phonons.