A method and apparatus are provided for analysing a fluid, especially an oil/water/gas mixture of unknown composition flowing in a pipeline (10). The apparatus can operate remotely, for example on the sea bed. A gamma-ray source (12) and a diametrically arranged scintillator (20) enable the density to be determined; while a fast neutron source (14) and a diametrically arranged germanium gamma spectrometer (22) cooled by a cryo-cooler (26) enable the concentrations of the elements present to be determined. The apparatus also includes a pulsed fast neutron source and a gamma detector to determine the flow velocity of the fluid. The apparatus enables the weight fractions of oil, water and gas to be derived.