PbO-P 2 O 5 glasses in which part of the PbO or P 2 O 5 has been replaced by Sb 2 O 3 were prepared by conventional melt-quench method and characterised by powder X-ray diffraction (XRD), infrared, X-ray photoelectron spectroscopy (XPS), 3 1 P magic angle spinning nuclear magnetic resonance, differential scanning calorimetry (DSC) and uv-visible optical absorption studies. (PbO) 1 - x (P 2 O 5 ) x glasses contain mainly two types of Q 2 structural units of phosphorus, which are ionically and covalently bonded to Pb 2 + . XPS studies of the glasses, where part of the PbO or P 2 O 5 has been replaced by Sb 2 O 3 , revealed the existence of both Sb 3 + and Sb 5 + in these glasses. 3 1 P MAS NMR studies of Sb 2 O 3 -substituted glasses suggested the formation of two types of additional Q 2 structural units of phosphorus which have Sb 3 + and Sb 5 + as next nearest neighbours in the network, thereby giving rise to different values of chemical shift. Sb 2 O 3 -containing glasses were found to be thermally more stable compared to PbO-P 2 O 5 glasses as revealed by the formation of crystalline Pb(PO 2 ) 2 phase at higher temperatures. E o p t values were found to decrease for the partial replacement of PbO or P 2 O 5 by Sb 2 O 3 in (PbO) 0 . 4 (P 2 O 5 ) 0 . 6 base glass.