[en] Research into instrumentation and experimental techniques for use in flowing two phase, gas-liquid systems is described. New and alternative techniques are being developed. At Harwell basic two phase flow and heat transfer research is performed, and there is considerable interest in the use of optical techniques to examine the structure of the fluid, and of infra red techniques for both temperature measurement and detailed flow investigations. At Winfrith research studies are directed towards a range of reactor situations of concern, and two phase instrument development is concentrated more upon measurement of bulk flow velocity, the emphasis being upon transient conditions appropriate to safety considerations. (U.K.)