An analysis of the fluorescence decay behavior of alexandrite emission, at higher dopant concentrations, has been undertaken for optical sensor application purposes. A double exponential decay behavior of the fluorescence emission has been observed, which differs from the single exponential behavior of alexandrite at lower concentrations of the active Cr 3+ ion. The origin of the phenomenon has been investigated both experimentally and theoretically and found to be produced by the joint contribution from the Cr3+ dopant in both the mirror, as well as in the inversion sites of that ion. The two lifetimes resulting, each having a different temperature dependence, both can be represented through the use of the configurational coordinate model which considers the effect of thermal quenching. The intensity ratio of these two aspects is seen to change regularly with the temperature within the important region for sensing of 273–773 K, and thus provides an alternative means to produce a calibrated fluorescence-based sensor. © 1997 American Institute of Physics.