The paper describes the measurement of the change of phase with distance of a 127.5-kc/s wave over a particular inhomogeneous land path and the deduction of the mean velocity.The measuring system made use of transmissions from stations of the English chain of the Decca Navigator System, and the path studied was that between the Lewes (Sussex) and Warwick stations (177 km). With the aid of a mobile phase-measuring equipment the difference between the phase of the signal from one transmitter and that of the signal from the other transmitter was measured with an accuracy of 0.25° at 25 points along the path. From these results and a knowledge of the exact position of the points it was possible to plot a curve showing the deviation of the measured phase from that calculated on the assumption of a velocity equal to that in free space.The mean velocity deduced for the path was 299 230 ± 12 km/sec.An examination was made of the measured change of phase along the path in relation to the profile and geological structure. Although there was no definite evidence of the phase of the wave being affected by the ground contours, the influence of the nature of the soil was very marked. A detailed study of the geological structure was undertaken and the electrical characteristics of each section of the path were assessed. A high velocity over ground of good conductivity and a low velocity over ground of poor conductivity was clearly shown.Comparisons between the measurements and the phase changes derived by a method based on theoretical curves of phase lag over homogeneous ground showed good correlation. They also confirmed the theoretical predictions of a characteristic phase disturbance at the boundary between sections of differing conductivity. It is concluded that this method of phase determination over an inhomogeneous path is one which offers a good approximation to the phase changes actually taking place.