Although the impedance of natural outdoor ground surfaces has been measured extensively at frequencies above about 200 Hz, measurements at lower frequencies are scarce and have been inaccurate. To overcome some of the difficulties at low frequencies, a two-microphone, phase difference technique to obtain the ground impedance has been adapted. A point source is suspended above the ground and the sound field is measured with two microphones along the vertical line below the source. The magnitude and phase of the reflection coefficient, and hence the impedance, is inferred from the variation of the phase difference between microphones as a function of height above the ground surface. Measurements were made in the range 25–300 Hz on flat, grass-covered ground, free of reflecting objects. The results obtained are consistent with recent seismic/acoustic coupling experiments and theoretical calculations based on layered poroelastic frame models for the ground. In particular, the measured results show evidence for fine structure expected from the layered nature of the ground.