Abstract: A method and apparatus for determining entrained gas-phase content of process streams comprising collection, isolation, and compression of a process sample, measurement of the compressive behavior, and calculation of the volume of entrained gas-phase by means of a thermodynamic equation of state. The sample is collected in a cylinder between a piston and a sealing plate. An actuator rigidly mounted on the piston force a piston rod into the contained sample where any gas phase is compressed. A pressure transducer measures change in pressure and another transducer measures linear travel of the piston rod to give information on volume change of the sample. The actuator travels with the piston at it is reciprocated by a first actuator to draw in or expel a sample. The apparatus is well adapted for automatic on-line measurement. A microprocessor operated control system for automatic operation is described.