Abstract Collision-induced dissociation (CID) of the proton-bound dimers of a set of pentapeptides (leucine enkephalin analogs) generated by electrospray ionization is studied as a function of collision energy under conditions of single collisions with argon. As the collision energy is increased, the abundances of the two protonated peptides become more similar, indicating an increase in internal energy deposition. The effective temperature ( T eff) of the cluster ions is calculated by the kinetic method and found to increase approximately linearly with collision energy. Knowing the fragmentation thermochemistry, the ion internal energy is characterized using the kinetic method. The partitioning quotient for the conversion of laboratory kinetic energy into internal energy for these cluster ions is 2% to 5% in the 50 eV to 200 eV collision energy range. Average relative standard deviations of multiple measurements of partitioning quotients are around 15% and are mainly due to uncertainties in ion abundance ratios. Unimolecular dissociation Rice–Ramsperger–Kassel–Marcus (RRKM) theory is used to calculate the relationship between the fragment ion abundance ratio and the total internal energy of the cluster ions. Comparison of these data with experiment allows the energy partitioning behavior to be characterized independently and more accurately. The partitioning quotient obtained in this way ranges from 2 ± 1.0% (uncertainty is the standard derivation of multiple measurements) to 5 ± 1.0%. These data are consistent with either an impulsive collisional activation mechanism or with collision complex formation.