Analytical expressions for distinguishing between different reaction and mass transport regimes in an isothermal microchannel reactor with first-order wall reaction are presented. These expressions are explicit functions of the Damkohler number (Da) and of the Graetz parameter (alpha Pe(m)/z), as well as of the degree of mass transport control (theta) which is usually set arbitrarily. The power law addition of contributions from fully developed and developing concentration profile conditions allows a correct description for all values of alpha Pe(m)/z. The scaling analysis suggests that the relative importance of mass transfer effects compared to reaction should be assessed by the rescaled Damkohler number Da*, defined with the correct scales for external and internal diffusion. It is also shown that a Da - alpha Pe(m)/z parametric map is appropriate for identifying the boundaries between regimes, which can be directly calculated from the dimensionless parameters in a very convenient manner. (C) 2011 Elsevier B.V. All rights reserved.