The densities of xBi(x)O(3)-(100-x)B2O3 binary melts (x = 0-100 mol %) have been measured systematically in the temperature range of 973 to 1373 K using the Archimedean double-bob method. In addition, the molar volume and coefficient of volume expansion have been determined from these density measurements. The density of melts in the Bi2O3-B2O3 binary system decreased with increasing temperature for all compositions, and increased with increasing Bi2O3 content. The molar volume calculated at 1273 K showed a minimum at around 20-30 mol% Bi2O3. The deviation of molar volume from ideality and the partial molar volumes of B2O3 and Bi2O3 single-component melts were calculated, and the relationships between these properties and Bi2O3 content were investigated. The Delta(MV) (deviation of molar volume from ideality) was negative in the Bi2O3-B2O3 binary melts and showed a minimum of -7.5 cm(3) (-17.3%) at 40 mol% Bi2O3 and 1273 K. The PMV (partial molar volume) of the B2O3 melt decreased with increasing Bi2O3 content, up to 70 mol% Bi2O3, and was constant above 70 mol% Bi2O3. The PMV of the Bi2O3 Melt was constant up to 30 mol% B2O3 (considered from the viewpoint of Bi2O3), and then decreased sharply with further addition of B2O3. The coefficient of volume expansion increased sharply up to 20 mol% Bi2O3 and decreased with further addition of Bi2O3 at 1273 K. From this result, it can be inferred that the temperature-induced structural changes taking place in the melts are greatest at 20 mol% Bi2O3.