We have made some improvements to the osmometer described previously (Colloid Interface Sci. 1981, 79, 548-566). Using the improved osmometer, we measured the osmotic pressures of aqueous bovine serum albumin (BSA) solutions at four pH values (4.5, 4.8, 5.4, and 7.4) and at four ammonium sulfate concentrations [(0.15, 0.50, 1.00, and 1.50) mol.L-1]. The osmotic second virial coefficients of BSA were calculated from the osmotic-pressure data. According to the molecular thermodynamic model (Fluid Phase Equilib. 2000, 168, 229-239), the electrostatic repulsion potential, attractive dispersion potential, and ion-excluded-volume potential have been calculated. The dependence of the three potentials on solution pH and on ionic strength is discussed.