We investigate numerically the steady-state properties of a mixing-controlled reaction between segregated reactants continuously inflowing-outflowing the annular region between counter-rotating coaxial cylinders in the Stokes regime. This system provides a prototypical example of an open nonchaotic flow generating steady-state kinematic mixing structures of arbitrarily fine lengthscales which can be characterized analytically. The kinematic interface structure is compared and contrasted to that of the reaction interface associated with the presence of a small but finite diffusivity. Results of accurate numerical simulations show that reaction efficiency and mixing performance are only weakly correlated with the global measure of both the kinematic and the reaction interface. Relevant information oil reaction regimes at low diffusivity can instead be obtained from the analysis of the spatial distribution of the kinematic interface, which controls the localization properties of unreacted species. The generality of the results is tested for several prototypical open and closed channel flows. (C) 2008 Elsevier Ltd. All rights reserved.