The mixing of different fluids or chemical species flowing through a microchannel is of great concern in many microfluidic applications, especially microchannel reactors. Flows in a microchannel are generally laminar, and mixing occurs due to molecular diffusion, which is a slow process. Mixing of solutions as a phenomenon is driven by the diffusion itself, but change of the channel shape results in the change of the mixing properties of particular microfluidic structure. This paper presents an experimental and theoretical investigation into the effect of channel axial cross-section shape on mixing and chemical reaction in microchannels. Three microchannels with uniform, converging, and diverging axial cross-sections were used to perform a liquid-liquid acid-base reaction that produces CO2 gas. Flow visualization demonstrated that the most intense chemical reactions (enhanced bubble formation) occurred in the diverging microchannel. Results of both qualitative mixing experiments using two reactive solutions and theoretical analysis indicated that the flow deceleration effect in the diverging microchannel significantly enhanced diffusive mixing in the lateral direction and, consequently, chemical reactions. From these results, it is concluded that a microchannel with a simple diverging cross-section can be used to develop microfluidic devices such as microchannel reactors.