Isotope effects on the loss of H and D are used to estimate the internal energy distribution in transient radicals produced by collisional electron transfer in the gas phase, as studied by neutralization–reionization mass spectrometry (NRMS). Experimental intermolecular and intramolecular isotope effects are compared to those calculated by RRKM theory on ab initio potential energy surfaces and convoluted with energy distribution curves. Fitting parameters in trial energy distribution curves provides widths and maxima that are compared with the energetics of ion and neutral formation. For collisional neutralization of 2-hydroxypyridinium, 3-hydroxypyridinium, and C(OH) 3 + cations by electron transfer from polarizable molecular targets, the most probable internal energy of the resulting transient radical is expressed as a simple sum of the precursor ion internal energy and the Franck–Condon energy acquired by vertical electron capture. This simple formula allows one to predict internal energies in transient neutral systems where a complete kinetic analysis of isotope effects is difficult or impractical.