The present study investigates experimentally the evolution of two-phase flow pattern and pressure drop in the converging and diverging, silicon-based microchannels with mean hydraulic diameter of 128 pm and CO2 bubbles produced by chemical reactions of sulfuric acid (H2SO4) and sodium bicarbonate (NaHCO3). Three different concentrations of 0.2, 0.5 and 0.8 mol/L of each reactant-at the inlet before mixing and 10 different flow rates from 1.60 x 10(-9) m(3)/s to 16.0 x 10(-9) m(3)/s are studied. Flow visualization is made possible by using a high-speed digital camera. It is found that the present design of the microchannel, with the inlet chamber, results in much more intensive chemical reactions in the diverging microchannel than that in the converging one. The void fractions at the entrance and exit regions and pressure drop through the channel are also measured. The results reveals that the presence of small void fraction, < 0.1, at the inlet may promote CO2 generation in the microchannel, irrespective of the channel is converging or diverging, indicating the agitation effects of bubbly flow in the microchannel. The increase of inlet concentration of reactants does not increase the pressure drop in the converging microchannel significantly, while the inlet concentration presents significant but mild effects on the pressure drop in the diverging microchannel. The two-phase frictional multiplier may be positively correlated with the mean void fraction in the channel linearly, and the data agree well with predictions from the correlations in the literature. (c) 2006 Elsevier Ltd. All rights reserved.