The glass formation regions in the system SeO2-Ag2O-B2O3 have been determined using the melt quenching method of evacuated silica ampoules. The structural units forming the amorphous network have been established by IR spectroscopy. The presence of SeO3 (nu = 820 cm(-1); 760-750 cm(-1)), BO3 (nu = 1340, 1270 cm(-1)) and BO4 (nu = 1050 cm(-1)) units has been confirmed. Crystallization of Ag2SeO3 only has been observed in a wide concentrate region near the glass formation boundary. A model explaining the unsatisfactory glass formation ability in the system investigated has been developed. It has been suggested that Ag+ ions are predominantly located near the selenite units, which stimulates the formation of isolated SeO3 groups. The transformation of BO3 into BO4 units is hindered by the absence of free Ag+ ions near the borate units. (C) 2007 Elsevier Ltd. All rights reserved.