Enzyme microreactors are used as research tools because of many advantages including low reagent consumption and the general notion that mass transfer restrictions are reduced. The use of microchannels can indeed shorten the characteristic mass transfer time but may also affect the productivity of the microreactor. To what extent mass transfer restrictions affect the reaction rate and the productivity is determined by parameters such as the enzyme properties, operating conditions, and dimensions of the microreactor. This article provides the correlations between these parameters for coflow enzyme microreactors obeying Michaelis-Menten kinetics. These correlations outline the design space based on reduced mass transfer restrictions and maximum productivity, respectively. The methodology that yields the design space provides a generic hands-on approach to optimally design coflow enzyme microreactors. We conclude that effectiveness is an important parameter for designing microreactors, but - depending on the reactor configuration - does not necessarily lead to optimized throughput. We demonstrate design windows to illustrate this effect and how to take advantage of it. (C) 2011 Elsevier B.V. All rights reserved.