Abstract: The nature of the flow pattern in a pipeline carrying both a liquid and a gas depends on the flow rates of the two phases. The flow pattern is characterized by measuring the void fraction or the average density along at least one chord across the pipeline (12) and by determining the distribution of liquid around the periphery of the pipe wall. The void fraction may be measured by the absorption of gamma rays from a source (14); while the peripheral distribution may be determined from the attenuation of ultrasonic plate waves propagating through sectors of the wall between transmitters (18) and receivers (20).