We investigate here the mechanism of collisionally-enhanced isotopic selectivity observed in infrared multiple photon dissociation (IRMPD) of vibrationally pre-excited CF3H by Boyarkin et al. (J. Chem. Phys. 118, 93 (2003)). For both the carbon-12 and carbon-13 isotopic species we measure the dependence of the IRMPD yield on the time-delay between the pre-excitation and the dissociation pulses at different dissociation frequencies as well as its dependence on the initial isotopic composition of the sample. The results reveal that the collisonal increase in isotopic selectivity originates not only from that of IRMPD itself, but also from the isotopic selectivity of vibrational energy transfer, with the latter making the major contribution under our experimental conditions. We suggest that the observed isotopic selectivity in collisional energy transfer arises from the difference in overlap between the absorption spectrum of the ν5 mode in the 12CF3H acceptor molecule with emission spectra of the same mode in the two isotopically different donors. Understanding the origin of this collisional effect has important implications for optimization of laser isotope separation processes.