Ultra-high conversions required in "ppm reactions" may result in a breakdown of the simple Langmuir approach for formulating reaction kinetics. Micro-kinetic analysis may provide a consistent basis for kinetic expressions covering a wide range of gas compositions. The impact of mass transfer restrictions changes in a complex way as concentrations approach zero, but the analysis of first-order reactions remains simple, Better knowledge of fluid mechanics is important in controlling maldistribution of flows and mixing of reactants. These aspects are illustrated by examples including selective catalytic reduction of NOx, catalytic oxidation, and methanation in ammonia plants.