Passive cyclonic gas-liquid separators (PCGLSs) have unique advantages compared with other types of gas-liquid separators. In this study, liquid distribution inside a horizontally installed PCGLS is investigated using electrical capacitance volume tomography (ECVT). The liquid distribution is measured under various gas and liquid flow rates. It is found that the inlet liquid flow rate is the dominating factor influencing the liquid distribution. The angular velocity of the liquid layer is found to be proportional to the liquid flow rate when no gas is injected. The center axis of the gas core is observed to be lower than the center axis of the separator cylinder, and it will drift towards the separator's center axis with increasing liquid flow rate. A mechanistic model is derived, and flow field parameters are solved analytically to qualitatively explain this phenomenon. The calculated gas core positions match the ECVT images well. (C) 2015 Elsevier Ltd. All rights reserved.