he gas and water content of crude oil flowing through a pipeline (1) is measured using a flow restriction (2) in the pipeline, a radioactive source (3) for providing separate radiation energies for each component to be measured, a detector (5) for detecting the radiation transmitted by the various components to generate a first set of signals, pressure gauges (8, 9) upstream of and in the flow restriction (2) for providing a second set of signals, and a signal processor (7) for correlating the signals to provide mass flowrate values indicative of the relative proportions of the components of the mixture. The temperature of the mixture in the flow restriction can also be measured (12) and fed to the signal processor (7) for ensuring that accurate values of flowrate are obtained when temperatures vary.