Microchannel T-junction configuration is used in chemical processing for passive mixing and reactions, because of its relatively easy control of the reaction environment. However, poor mixing and low reaction rate are some of the main drawbacks of this design. This study proposes mass-transport enhancement in a microchannel T-junction by utilizing various configurations of coiled base channel design (e.g., conical spiral, in-plane (flat) spiral, and helical spiral coils). Their mass-transport performance is investigated numerically and compared with that of the conventional straight microchannel T-junction. Laminar flow of gas flow with reactions is investigated using a three-dimensional computational fluid dynamics (CFD) model. Four different microchannel designs, three different channel Reynolds numbers, and three different helical coil diameters are investigated. The coils are made of a square cross-section tube. The results indicate that the coiled base channel displays improved mixing and conversion rate of the reactant. Finally, advantages and limitations of each design are discussed in the light of present numerical results.