In this work thermal lens spectrometry is applied to determine the fluorescence quantum efficiency, g, of Nd 2 O 3 -doped low silica calcium aluminosilicate glasses as a function of the temperature. The experiments were performed in the temperature range between 22 and 180 °C. The results showed that the decrease in g observed with increase in the temperature of the samples was larger for the larger Nd 2 O 3 -doping concentrations. This observation indicates that the non-radiative relaxation processes induced by Nd–Nd ion interactions may be the dominant mechanism responsible for the g reduction. The results indicate that the thermal lensing technique is a useful tool to evaluate the effect of the active medium temperature in the performance of laser systems. Ó 2002 Elsevier Science B.V. All rights reserved.