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The initial stage of downward co-current annular flow was studied using two optical techniques: brightness-based laser-induced fluorescence technique and shadow technique. The experiments were conducted in a circular pipe and in a rectangular duct. The working area was represented by the first few centimetres from liquid inlet; the latter was organised as a tangential slot. The process of formation of disturbance waves was found to consist of three stages: formation of regular initial two-dimensional high-frequency waves; fragmentation into localised irregular three-dimensional waves and formation of large-scale quasi-two-dimensional disturbance waves. All the stages occur closer to the inlet at higher gas velocities and lower liquid flow rates. It was found that the initial two-dimensional waves appear at the initial area of thick film in the vicinity of the inlet. Spatiotemporal measurements were conducted and …