Abstract: Multiphase fluid flow, such as a mixture of oil, water and gas, is measured by a system which includes one or two densitometers for measuring the multiphase fluid density flowing through a flowline or conduit and a flow restriction, pump, expander or heat exchanger interposed in the flowline between the densitometers to effect a pressure or temperature change in the multiphase fluid. Densities, pressures and temperatures are measured on the upstream and/or downstream sides of the flow restriction, pump, expander or heat exchanger and the measured values of density, pressure and temperature are used to obtain the flow rates of the respective phases in the flowstream.