This paper investigates the hydrodynamics of the two-phase liquid-gas Taylor or 'Slug' flow regime commonly encountered in micro and mini-channel flows. The primary focus is on understanding the mechanisms that lead to variations in the thickness of deposited liquid films and their effect on flow dynamics. A variety of test fluids are utilised over a range of experimental velocities, leading to significant variations in Capillary and Reynolds number. Slug flow regimes and liquid films formed under these conditions are observed using high speed optical microscopy with refractive index matching used to account for tube external curvature. An analytical correction procedure is also employed to correct for refraction errors due to light transmission through differing depths of liquid film on the interior tube surface, since each fluid has differing refractive index. Experimental data is found to correlate well with an existing model for film thickness when bubble velocity is used in evaluating Capillary number. While this model was developed for Capillary numbers up to 0.3, the present data significantly extends its application range to Capillary numbers as high as 1.9. A significant difference between the bubble and mean two phase velocity was also observed and related to the ratio of cross-sectional areas between the channel and the bubble which is shown to be valid when the viscosity ratio between phases is sufficiently high. Furthermore, a new model is put forward for predicting bubble velocity in Taylor flows when the viscosity ratio between phases is low and velocity within the film becomes appreciable. Expansion of the gaseous phase due to pressure drop along the channel was also shown to contribute to a significant increase in mean two phase velocity and this was successfully accounted for using pressure drop correlations and ideal gas relations. Overall, the study provides a greater understanding of liquid film deposition and bubble dynamics in Taylor flows and should enable future studies to more accurately model parameters of interest in microfluidic devices incorporating such flows. (c) 2013 Elsevier Ltd. All rights reserved.