This paper discusses ultrasonic measurement of unsteady flow. After a brief description of steady pipe flow, the measuring process is analysed from a signal-processing point of view. It can be described as a procedure consisting of three consecutive steps: sampling of the continuous-time flow velocity and thereby creating a discrete-time variable, a Moving-Average (MA) filtering operation, and finally sampling of the filtered signal at a rate of 1Hz. These three steps are described mathematically. Measurements have been conducted with a 12-inch Gassonic-400 at a reciprocating compressor station of the Tenneco Gas Pipeline Company, Houston, Texas, USA, and with a 20-inch Q.sonic at the Bernoulli Lab of N.V. Nederlandse Gasunie in Westerbork, The Netherlands. It is concluded that under conditions of pulsating and fluctuating flow the Gassonic-400 and Q.sonic ultrasonic flow meters results indicate within specification.