Three commercial fuels were studied: biodiesel (based mainly of the fatty acids methyl esters of rapeseed oil), diesel oil Ekodiesel Ultra (standard petroleum diesel oil with sulphur content less than 10 mg/kg), and ON BIO 10 (blend of 20 vol.% of biodiesel with 80 vol.% of standard petroleum diesel oil with sulphur content less than 10 mg/kg). The speeds of sound were measured within the temperatures from 293 to 318 K and at pressures from 0.1 to 101 MPa. The densities and heat capacities were measured under atmospheric pressure in the temperature range from 273 to 363 K and 283 to 359 K, respectively. Using the experimental results, the physicochemical properties such as: density, isentropic bulk modulus, heat capacity, and isobaric thermal expansion were calculated in the same temperature and pressure range as the speed of sound was measured. The results obtained show that although the bulk modulus of ON BIO 10 is higher than that of diesel oil Ekodiesel Ultra over the whole pressure range, the difference is rather small and can be compensated by temperature. Isobaric thermal expansivity of biodiesel decreases with pressure slightly less than that of the diesel oil Ekodiesel Ultra. It is approximately independent of temperature and composition of the fuel at pressures 40 5 MPa. (c) 2007 Elsevier Ltd. All rights reserved.