The phase distortion of a sonar pulse reflected at the interface of two fluid media is determined theoretically, and curves of the reflected pulse are presented for a number of phase shifts. The incident pulse is one cycle of a sine wave. There is no distortion for phase shifts of 0, π, and π/2, although in the latter case the pulse is inverted. For intermediate phase shifts, the pulse is distorted and shows both a “precursor” and “tail.”