The speed of sound in the high molecular weight normal paraffins n-C28H58 and n-C36H74 was measured using a pulse technique operating at 3 MHz. The measurements were carried out at pressures up to 150 MPa in the temperature range 353.15 K to 403.15 K. The data were used to determine various thermophysical properties such as density, isentropic and isothermal compressibilities up to 150 MPa with the help of additional density measurements performed at atmospheric pressure.