This paper describes a technique that can be used to analyze the reactions that take place in a reactor via mass balances, a graphical representation and interpretation of basic thermodynamic principles. This allows for the chemical species taking part in the reactions being considered, to create lines that define attainable boundaries in a GH plot. The end result is an attainable area that satisfies the conditions for an attainable region in a GH plot. The simultaneous methanol synthesis from syngas with the Water Gas Shift (WGS) reaction is used to illustrate this approach. Considering the investigation, one can readily see that at higher temperatures the reaction was not feasible thermodynamically at 1 bar, but at increased pressure the reaction could again become favorable and thermodynamically feasible. This paper also shows that the introduction of either water or CO2, or both, to the feed opens up the mass balance region, resulting in WGS activity and generating more reaction path alternatives. Again, the change in Gibbs free energy across the reactor and the reaction pathways leading to product are interlinked. Crown Copyright (C) 2012 Published by Elsevier B.V, on behalf of The Institution of Chemical Engineers. All rights reserved.