We used optical methods such as Laser induced Fluorescence (LIF) and confocal Laser Scanning Microscopy (LSM) to characterize gas-liquid phase distribution in rectangular microchannels. Using a 2 m long microchannel with a hydraulic diameter of 200 mu m enables the precise measurement of important parameters such as liquid slug length, bubble length, pressure drop and film thickness at the wall as well as in the corner of the microchannel for low Capillary numbers (Ca) ranging from 2 x 10(-4) to 1 x 10(-2). This range of Ca was obtained by using different fluid pairs such as ethanol, water and different concentrated aqueous solutions of glycerol in combination with nitrogen. The investigated segmented gas-liquid flow (Taylor flow) was very stable, meaning a standard deviation of the measured gas bubble lengths of below 5% and 10% for the liquid slug length, respectively. Using higher viscosities than 10 mPas resulted in an unstable flow - mainly due to the pressure drop. For viscosities in the range of 10 mPas the flow pattern changes: the slug lengths are much longer than the channel diameter. We demonstrate that the film thickness in the corner slightly decreases with Ca. For low Ca the film thickness at the wall stayed nearly constant. We observe a contribution of the film in the corner to the total film area of about 70%. The obtained gas bubble length depends mainly on the viscosity and on the pressure at the gas inlet. As the measured liquid holdup is in all cases higher than the theoretical holdup defined by the flow rates, the bubbles move faster than the liquid phase. The measured values are compared with literature data. (C) 2008 Elsevier Ltd. All rights reserved.