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 spectral and statistical approaches were applied to study the characteristics of the initial waves at the linear growth stage. The obtained characteristics were compared to the linear stability calculations; the comparison showed satisfactory agreement for the frequencies and velocities of the initial waves after taking into account non-equilibrium film thickness at the inlet. (C) 2018 Elsevier Ltd. All rights reserved.