The effect: of composition on the thermal properties and the spontaneous emission probabilities of various 0.5 mol% Tm2O3 containing (1 - x)TeO2 + (x)LiCl glasses were investigated using differential thermal analysis (DTA) and ultraviolet-visible-near-infrared (UV/VIS/NIR) absorption measurements. DTA curves of the samples were obtained in the 23-600 degreesC temperature range with a heating rate of 10 degreesC/min. The value of the glass transition temperature T-g and the crystallization temperature T-c were found to vary with the glass composition. Melting was not observed for the glasses containing less than 50 mol% LiCl in this temperature range. However, a melting peak was observed at T-m = 401 degreesC for the glasses having higher than 50 mol% LiCl, which were also found to be moisture-sensitive. Absorption measurements in the UV/VIS/NIR region were used to determine spontaneous emission probabilities for the 4f-4f transitions of Tm3+ ions. Six absorption bands corresponding to the absorption of the (1)G(4), F-3(2), F-3(3), F-3(4), H-3(5), and H-3(4) levels from the H-3(6) ground level were observed. An integrated absorption cross section of each band, except that of H-3(5) level, was found to vary with the glass composition. The role of the Judd-Ofelt parameters and therefore the effect of the glass composition on the radiative transition probabilities for the metastable levels of Tm3+ ions are discussed in detail.