This paper was prepared for presentation at the 47th Annual Fall Meeting of the Society of Petroleum Engineers held in San Antonio, Tex., Oct. 8–11, 1972. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by who the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. A primary research facility has been constructed by EPS* in their laboratory at Clamart, France, for the investigation of multiphase-flow phenomena in vertical pipes. This flow loop is capable of circulating pipes. This flow loop is capable of circulating multiphase combinations of gas (air), oil, and/or water, at controlled rates through vertical pipe of variable cross section. The first application of the Clamart flow loop is in the design, development, and evaluation of new flow-measurement devices for Production Logging services. A second and equally important application has been the study of multiphase flow phenomena per se. In Production Logging measurements involving two-phase flow, the crux of the interpretation problem is the estimation of slip velocity. (Slip velocity is the rate at which the lighter phase rises relative to the heavier fluid.) Investigations have been carried out to determine the effects of fluid properties and flow rates upon slip-velocity determination in two-phase flow. The bubble velocity-holdup-slip velocity relationships are examined, and comparisons are made of Clamart experimental findings with other models and descriptions of two-phase tow. In particular the dependence of bubble-rise velocities on interfacial tension and of slip velocity on holdup are exhibited. Considering the complexity of multiphase-flow phenomena, the agreement between mathematical models and experimental data is quite good. P. 1