Thermophysical properties such as speed of sound, density, and isentropic and isothermal compressibilities have been obtained from ultrasonic speed measurements on two synthetic systems which are representative of distillation cuts with high bubble points (T = 520 K and T = 573K, respectively). The first system (composition expressed in mass fraction), (0.40C(13)H(28) + 0.35C(13)H(26) + 0.25C(13)H(20)), was investigated at pressures from (0.1 to 150)MPa in the temperature range T = (293.15 to 373.15)K, whereas the second one, (0.40C(17)H(36) + 0.35C(16)H(32) + 0.25C(16)H(26)), was investigated at pressures up to 100 MPa in the temperature range T = (293.15 to 373.15) K.