Ultrasonic measurements on liquids at very high pressures and an accurate self‐consistent ultrasonic method for the determination of the equation of state of liquids at very high pressures are described. The liquid sample is contained in a polyethylene‐lined piston and cylinder device. The method described requires only the measurement of the transit time of an ultrasonic pulse through the liquid as a function of pressure. The difficulties and errors connected with a direct volumetric determination are thus avoided and a high accuracy is achieved. A computation of the uncertainties in the determination of the bulk moduli, the density and the velocity of sound is presented. Experimental results are given for water at 22°C and pressures up to 12.6 kbar. Data for the density of water reported by P. W. Bridgman and by L. H. Adams, and for the velocity of sound reported by Holtonet al.are found to be in good agreement with the present work.