A pair of optical void sensors and a high-speed video camera were used to investigate the evolution of adiabatic gas-liquid two-phase flow in a long microchannel. Experiments were conducted with a 1676-mm-long, circular microchannel with an inner diameter of 100 mu m. Two-phase flow patterns, void fraction, and velocities of gas plug/slug and liquid slugs were measured at different axial locations between the gas-liquid mixer and microchannel exit. The pressure decreased linearly in the first half of the microchannel, and more rapidly and nonlinearly in the second half of the test section. As a result, the flow accelerated significantly in the second half of the microchannel such that the void fraction and liquid slug velocity increased nonlinearly. The measured mean void fraction and mean velocity of liquid slugs also agreed well with the homogeneous flow model predictions when the liquid flow rate was constant and the mass velocity of the gas was low.