Helically coiled tubes have excellent compactness, heat transfer efficiency, thermal stress adaptability and two-phase flow stability. The increasing application of two-phase flow in helically coiled tube devices necessitates a deep knowledge of phase distribution features for different flow regimes as well as flow regime transitions characteristics. This paper identified the flow regimes of air-water adiabatic two-phase flow in three helically coiled tubes using a double-sensor conductivity probe and high speed camera. Similar to the inclined straight tube, a total of six flow regimes were found in the helically coiled tubes. The influence of centrifugal force on the phase distribution was analyzed for each regime. A set of straight-forward flow regime transition criteria were proposed and then validated by benchmark experiments conducted in an inclined straight tube. These criteria were then used to generate flow regime maps based on the experiments with liquid superficial velocities ranging from 0.03 to 4.12 m/s and gas superficial velocities ranging from 0.03 to 16 m/s. Compared to the horizontal and inclined straight tubes, the helically coiled tube experienced shrinkage in the bubbly flow region; also, wavy and annular flows appeared at slower gas superficial velocities. (C) 2016 Elsevier B.V. All rights reserved.