The isothermal bulk modulus of compressibility is, together with viscosity, and density, one of the properties affecting diesel injection processes. This property was measured by means of a specific apparatus that compresses a fuel sample in a high-pressure closed bottom capillary tube (a pycnometer) and by observing the height change in the column of fluid as pressure is increased over a range of pressures from 3 to 33 MPa. The bulk modulus of a conventional diesel fuel, a soybean oil derived biodiesel, and a hydrotreated soybean oil diesel fuel, together with that of their ternary blends, was measured. Water served as the calibration fluid, and measurements were made at 38 degrees C. All the fuels tested displayed substantially lower isothermal bulk modulus than water. There are observed differences between these fuel samples, with biodiesel displaying much lower compressibility than both the conventional diesel fuel and the hydrotreated oil. Although most of the blends showed bulk modulus values proportional to their volume fractions, some nonlinear effects were found when diesel fuel was involved in the blends, and particularly, a synergistic effect was found in blends with diesel and hydrotreated oil. An optimized correlation has been obtained to predict bulk modulus from the volume proportions of the three fuels tested.