The process of formation of disturbance waves in annular gas-liquid flow was studied using three-dimensional configuration of brightness-based laser-induced fluorescence technique in two ducts of different geometry in the proximity of the inlet. The process was found to consist of three stages: formation of initial two-dimensional high-frequency waves; fragmentation into localised chaotic 3D-waves and formation of large-scale quasi-2D disturbance waves. A method of quantitative characterisation of the degree of transverse coherence of film surface was developed and used to estimate the coordinates of the borders of the three stages. All the stages occur closer to the inlet at higher superficial gas velocities and lower liquid flow rates. It was found that the shape of the duct does not exert significant influence of the distance of existence of two-dimensional initial waves, whilst the disturbance waves are formed closer to the inlet in the circular duct.