The sound pressure in a one‐dimensional duct contains amplitude and phase components, both of which vary with position. The spatial variation of phase is examined in this paper and is shown to be closely related to the relative flow of incident and reflected acoustic energy. A technique that determines the acoustic reflection coefficient of the duct termination by measuring the maximum rate change of phase with position is discussed. For the measurements of phase variation a single moveable probe microphone is used with a phasemeter. Good agreement has been obtained between this method and the more usual impedance tube method, which considers the amplitude component of sound pressure. The phase measurements, though, can be applied equally well to ducts of conical, rather than uniform, cross section, and require a relatively small length of duct in which to translate the probe (as little as 1/20 wavelength, for moderate reflectivities). Some preliminary measurements have been made in human ear canals. The eardrum reflection coefficients obtained in the 10‐ to 15‐kHz range are between 0.6 and 0.85, somewhat higher than those obtained by other methods.