Liquid phase residence time distributions (RTD) are reported by employing a novel experimental method of tracking a radioactive tracer particle during single phase and two-phase (gas-liquid) flows through a horizontal helical coil. Liquid and gas phase Reynolds numbers (N-Re,N- (L) and N-Re,N- G) were varied in the ranges from 1061 to 23,150 and 130 to 100,000 respectively. As part of this work, we tracked the entry and exit of the tracer particle in the coiled flow structure using an array of strategically placed scintillation detectors. From these experiments, it became possible to extract the residence time distribution (RTD) by enumeration of the trajectories of the tracer particle through the flow system. The investigations have resulted in explaining mixing performance for gas-liquid (two-phase) flow in coiled geometry, based on the trends in liquid phase Peclet number (N-pe), over varying liquid and gas phase velocities. (C) 2016 Elsevier Ltd. All rights reserved.