The internal energy distribution P(Eint) of ions emitted in an electrospray (ESI) source interfaced with a sectormass spectrometer is evaluated by using the experimental survival yield (SY) method including the kinetic shift. This method is based on the relationship between the degree of fragmentation of an ion and its amount of internal energy and uses benzylpyridinium cations due to their simple fragmentation scheme. Quantum chemical calculations are performed, namely at G3(MP2)//B3LYP and QCISD/MP2 levels of theory. The results show that the internal energy distribution of the ions emitted in the ESI source interfaced with a sector analyzer is very narrow. The MassKinetics software is used to confirm these observations. The P(Eint) is the parameter that allows to fit the experimental SY of each substituted benzylpyridinium cation with theoretical mass spectra generated by the MassKinetics software. The resulting internal energy distributions are similar to the ones obtained with the experimental SY method. This indicates that in the present experimental conditions, P(Eint) cannot be compared with a ‘thermal-like’ Boltzmann distribution. In addition, it appears that with the sector analyzer, increasing the collision energy in the first pumping stage of the ESI source does not correspond to awarm-up of the produced ions.