The thermal-mass spectroscopy coupling technique (TG-DTA-MS) was used to study the thermal decomposition behaviour of the transition from xerogels to glasses for the gradient refractive index materials SiO2-PbO-B2O3 with added dimethyl sulphoxide (DMSO). Compared with the TG-DTA-MS results for C2H5OH-system xerogels, the MS curves for DMSO-system xerogels show (in addition to CH3+ (m/z, 15)) the CH3O+ (mit, 31), C2H5O+ (m/z, 45), [(CH3)(2)SO](+) (mz, 78), etc., positive-ion mass spectrum peaks. This indicates that adding DMSO enables many residual OR groups to remain in the forming xerogels. This was beneficial for the condensation of the incompletely hydrolysed product and for preventing cracking of the xerogels. A preliminary study on the mechanism of thermal decomposition for DMSO-system xerogels is also given in this paper.