We present a generally applicable theoretical model describing excited-state decay lifetime analysis of metal ions in a host crystal matrix. In contrast to common practice, we include multi-phonon non-radiative transitions competitively to the radiative one. We have applied our theory to Co2+ ions in a mixed AgCl0.5Br0.5 crystal, and as opposed to a previous analysis, find excellent agreement between theory and experiment over the entire measured temperature range. The fit predicts a zero absolute temperature radiative lifetime τr ad(0) = 5.5 ms, more than three times longer than the measured e ective low-temperature one τe ff (0) = 1.48 ms. Furthermore, the fit configuration potential dissociation energy has been estimated as D = 2500 cm−1 and the lattice vibrational cuto frequency as ωco = 180 cm−1. We have experimentally verified the latter by optical reflection measurement in the far-IR.