Mode-mismatched thermal lens (TL) measurements in Cr3+-doped fluoride crystals (LiSrAIF6 and LiSrGaF6) are reported. A nonlinear increase of the TL signal, and decrease of quantum efficiency, with increasing excitation power was observed and attributed to energy-transfer upconversion (ETU). Assuming an upconversion rate that is proportional to the excited-state population, Wup = gammaN(e), the theoretical model developed fits the experimental data well. The ETU parameters (gamma) were determined with unprecedented accuracy for low Cr concentrations. Thermo-optical parameters (K, ds/dT) were also obtained.