In the present paper, the characteristics of adiabatic two-phase flows in a horizontal circular microchannel have been reported. In experiments, in order to determine the effects of fluid properties on the flow characteristics, distilled water and aqueous solutions of ethanol having four different mass concentrations were used as the working liquids, while nitrogen gas as the gas. The nitrogen gas and one of the four liquids were injected through a T-junction type mixer to the test microchannel made of fused silica. To know the effects of flow contraction at the channel inlet, two mixers of different inner diameters of DM
= 250 μm and 500 μm were used at a fixed microchannel diameter of D
= 250 μm. Bubble velocity data and/or void fraction data were correlated with the well-known drift flux model. The distribution parameter, C0
, in the model was found to depend on both the liquid properties and the flow contraction, i.e., C0
increased with increasing of the liquid viscosity and/or decreasing of the surface tension, and was higher for the flows with the contraction. The pressure drop data were correlated with the Lockhart-Martinelli method. The two-phase friction multiplier, φL2
, was lower for the flows with the contraction.