Basic thermodynamic analyses can provide in-depth knowledge of the mechanism of chemical reactions. This short review recalls first the definitions of the approach-to-equilibrium eta, the reaction quotient Q and the thermodynamic equilibrium constant K. Thereafter, four case studies specifically dealing with gas-phase heterogeneously catalyzed reactions are reviewed: (i) alkane hydroisomerization, (ii) NO oxidation during the selective catalytic reduction of NOx with propene, (iii) the steam reforming of methanol and (iv) ethanol condensation to butanol. These examples illustrate in different manners how a reaction mechanism can be supported or rejected based on rather simple analyses of the concentrations of reactants and products. While the examples used here are all referring to catalyzed reactions, it must be emphasized that the method can be applied to non-catalytic systems. (C) 2014 Elsevier B.V. All rights reserved.