Experimental speeds of sound and densities are presented for the liquid phase of poly(propylene glycol)-425. The measurements were carried out along nine isotherms from (293.15 to 373.15) K at pressures from saturation condition up to 100 MPa. The speed of sound was measured by a pulse-phase echo ultrasonic device at a frequency of 1 to 5 MHz with an uncertainty of +/-0.2%. The density was measured by an acoustic piezometer constructed by the authors with an uncertainty of +/-0.3%. The experimental results were used to calculate isobaric thermal expansion coefficient up and isentropic compressibility k(S).