A very interesting phenomenon occurs during the simultaneous flow of gas and liquid through a vertical concentric annulus. Although the geometry is axially symmetric, the two phases distribute themselves asymmetrically over a wide range of phase velocities. The present work has attempted to investigate this asymmetric phase distribution across a cross-section through extensive experimentation by using the conductivity probe technique. The probe signals and their probability density function analysis have indicated that this feature is initiated in the bubbly-slug flow regime and persists in the entire range of slug and slug-churn flow patterns. The interfacial configuration is symmetric in bubbly, fully formed churn and the annular flow patterns. Finally, the reason for this asymmetry has been attributed to the formation of cap and Taylor bubbles which are never symmetric with respect to the axis since they always form open annular rings. (C) 2000 Elsevier Science Ltd. All rights reserved.