Given that the values of + and are not intrinsically related to the relative thermodynamic stability of Z-17 and E-17 there is no reason for their ratio at photostationary state (PSS) to accord with the corresponding equilibrium constant. This means that all 53 photochemical exchange processes under continuous illumination automatically correspond to a non-equilibrium steady state. Based on this fact, the key question to ask of a network that includes a photochemical step is whether it is coupled to the process of interestthe same question that we advocate should be asked of all chemically driven systems.