The speed of sound in liquids pentadecylcyclohexane (C21H42) and nonadecylcyclohexane (C25H50) was measured by means of a pulse technique operating at 3 MHz. The measurements were carried out at pressures up to 150 MPa in the temperature range (303.15 to 383.15) K. Additional density measurements were performed up to p = 60 MPa in the same range of temperature. From both measurements, the density was evaluated up top = 150 MPa and the isentropic compressibility was determined in the same pressure-temperature domain. The full density data were represented by a Tait-like equation in order to calculate the isothermal compressibility. (C) 2003 Elsevier Ltd. All rights reserved.