This paper examines the augmentation of heat and mass transfer due to dual clearances on cylindrical pin fins, relative to a channel between parallel plates, in mini/microchannel reactors at low Reynolds numbers. In this work, diffusion limitations to heat and mass transfer in smooth-walled mini/microchannel reactors were minimized by the implementation of microcylinder pin fins with dual clearances that, (1) promote the production of instabilities in the wakes that enhance mixing and (2) reduce the viscosity dominated regions at pin-wall interfaces. A smooth catalyst coating is assumed on all exposed surfaces of the microchannel interior walls and pin fins. Due to the analogy of heat and mass transfer, augmentation of the Nusselt number is equivalent to the augmentation of the Sherwood number. Heat transfer augmentation is investigated in air (Pr = 0.705) at dual clearances ranging from 0 to 0.4 of the channel height and Reynolds numbers from 10 to 600. The pin fins and the clearance augmented the heat transfer coefficient by a factor of 4.0. The combination of the augmentation of the heat transfer coefficient and the increase in the surface area, by the clearances, results in an increase in the conductance over a plane channel, by a factor of 7.1. The results are extendable to overcoming laminar diffusion with laminar periodic wakes of fuel vapors such as methanol vapor in air where Scfuel ∼ Prair. For turbulent wakes impinging upon downstream pins, the results can be extended to fuel vapors with (Scfuel)turb ∼ (Prair)turb. A large eddy simulation (LES) approach was used in this study.