At equilibrium, the relationship K = k(f)/k(r) between the equilibrium constant and the rate constants of an elementary reaction is a consequence of the principle of detailed balancing. Out of equilibrium, it remains valid for most elementary reactions. In general, it is not valid for complex reactions out of equilibrium but its validity for the elementary steps of the mechanism may have important consequences for the interpretation of the experimental results. A related criterion of thermodynamic consistency of reaction mechanisms is the absence of null cycles defined as sets of different reactions taking place under given conditions and leading to no net stoichiometric change.