A new glass system SnO-MgO-P2O5 with low viscosity has been developed by a melt-quenching method. Formation, thermal properties, and chemical durability of these glasses have been investigated. For a constant P2O5 concentration, the glass formation ability is enhanced with the increasing Sn/(Sn + Mg) ratio. The glasses exhibit low glass transition temperature (T-g = 270-400 degrees C), low dilatometric softening temperature (T-DS = 290-420 degrees C), and high thermal expansion coefficient (CTE = 110-160 x 10(-7) K-1). With the increasing Sn/(Sn + Mg) ratio, T-g and T-DS decrease, and CTE increases. When Sn/(Sn + Mg) ratio is varied, the relationship between chemical durability and thermal properties of the present glasses is not consistent with what expected in general cases. It is noted that the glasses with 32-32.5 mol% P2O5 exhibit excellent chemical durability and tunable T-g, T-DS, and CTE (by varying Sn/(Sn + Mg) ratio).