In this work, monolith reactor optimization is carried out. A 3-D monolith reactor model is first developed for a plug flow, isothermal, reactor featuring first order irreversible volumetric and surface reactions, then solved analytically using a series-based solution method, and finally optimized by establishing a number of mathematical model properties, and carrying out a broad search of the resulting reduced two-dimensional design parameter space. It is shown that maximum conversion is attained at a fluid velocity upper bound determined by the channel's width, height, and capital cost to compression cost ratio. For reactors featuring only a surface reaction, maximum conversion is also attained at the maximum allowable channel width, and the minimum allowable channel hydraulic diameter. The optimum reactor residence time is inversely proportional (proportional) to the channel's optimum hydraulic diameter (the reactor's capital cost to production ratio). (C) 2019 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.