A study of the transient displacement of gas and liquid phases in microchannels is conducted. A fluid dynamics model for bulk fluid displacement in 200 mu m channels is developed and validated with the data from an air-water flow visualization study performed on glass microchannel test sections of 200 mu m diameter using high speed videos. Interface velocities, void fractions and film thicknesses are determined by performing image analysis on the videos obtained. The bulk fluid displacement models for fluid velocity are found to be in good agreement with the data for the displacement of gas by liquid, with a mean error of 11%. During the displacement of gas by liquid, a single liquid slug cleanly displaces the gas in the channel with little interaction at the liquid-gas interface. For the displacement of liquid by gas, the mean model error is 20% and the flow patterns observed are dry-wall, thin-film, ring-film, intermittent, and rivulet flows. (C) 2017 Elsevier Ltd. All rights reserved.