The hydrodynamic flow behavior in a Gas-Liquid Cylindrical Cyclone (GLCC) compact separator is studied experimentally and theoretically. New experimental data are acquired utilizing a 7.62 cm I.D, 2.18 m high, GLCC separator for a wide range of operating conditions. Investigated parameters include three different inlet geometries (5.08 cm I.D single, 7.62 cm I.D single and 7.62 cm I.D dual inlets), four different liquid viscosities (1, 2.5, 5 and 10 cps), three system pressures (101.3, 273.6 and 487.2 kPa), and the effect of surfactant, The measured data comprise of equilibrium liquid level, zero-net liquid flow holdup and the operational envelope for liquid carry-over. The data are utilized to verify and refine an existing GLCC mechanistic model. Comparison between the modified model predictions and the experimental data show a very good agreement. (C) 2000 Elsevier Science Ltd. All rights reserved.