Mixed potassium-lead borophosphate glasses were prepared and studied in two compositional series xK(2)O-(50-x)PbO-10B(2)O(3)-40P(2)O(5) and xK(2)O-(50-x)PbO-20B(2)O(3)-30P(2)O(5) with x = 0, 10, 20, 30, 40 and 50 mol% K2O. The replacement of lead by potassium decreases the density and increases the molar volume of these glasses. On the other hand both glass transition temperature and chemical durability decrease. The observed changes in the properties of these glasses are explained as due to changes in their structure and ionicity of chemical bonds between cations and the anionic network of the glass. The major role is played by differences in the space occupied by cations, differences in electronegativity and the field strength of the corresponding cations. (C) 2004 Kluwer Academic Publishers.