Abstract Glass-forming tendencies of melts in the systems (alkali oxide or alkaline earth oxide)-Nb 2O 5Ga 2O 3 were examined by an ordinary crucible-melting technique. The glass-forming tendency increased with increasing radius of alkali or alkaline earth ion in the respective groups. Clear glasses were obtained on a practically useful scale in the systems (K 2O or Cs 2O)Nb 2O 5Ga 2O 3 and (SrO or BaO)Nb 2O 5Ga 2O 3. The infrared absorption spectra indicated that the Ga 3+ ions in the glasses are tetrahedrally coordinated with oxygen ions. The glasses showed high optical transmissions from the ultraviolet region of 0.3 μm in wavelength to the infrared region of 7 μm, except for a region near 3 μm. The absorption near 3 μm, which is attributed to OH vibration, could be eliminated by replacing part of the carbonate in the raw materials with a fluoride and melting the mixture of raw materials in a dry N 2 gas atmosphere. The glass-forming tendencies of the melts and the optical transmissions of the glasses were discussed in terms of the glass structure.