The structure of tin-borate and tin-boratephosphate glasses has been examined with diffuse reflectance infrared (DR-IR) and Raman spectroscopy. The basic network structure for these glasses is described as well as the positioning of tin in the network. Data suggests that the amount of phosphate present in the glass regulate the glass forming properties of tin. With borate as the dominating glass forming oxide, SnO acts like a glass former, but with increasing amount of phosphate SnO instead tends to behave as a network modifier. (C) 2003 Elsevier Science B.V. All rights reserved.