An optical method is developed for a 2.95 mm inner diameter circular mini-channel to estimate liquid film thicknesses. The greyscale pictures obtained with a high-speed camera are processed to determine the liquid-vapour interface positions for annular and intermittent flows. The experiments are thus performed for a large range of flow conditions. The saturation temperatures tested ranged from 20°C to 100°C in steps of 10°C and the mass velocities are 50, 100, 200, 300 and 400 kg m−2 s−1. A new parameter ranging from 0 to 1, the symmetry, is defined to account for the level of non uniformity of liquid distribution around the tube perimeter. New experimental data are presented (270 data points), that cover a range of symmetry parameter from 0.35 to 1.00. These data are merged with those available in the literature (406 data points with symmetry parameter from 0.71 to 1.00). A sensitivity analysis of the dimensionless numbers of major influence is run and a new correlation is proposed, that enables to predict over 90% of the data points in an error bandwidth of 10%. This correlation is proposed as criterion for asymmetry.