The analytical solution achieved for the simplest photoreaction and the discovery of Φ-order kinetics, as described in the previous chapter, raise interest towards studying more photokinetic systems under non-isosbestic irradiation. The central question here is whether closed-form integrations can be achieved for other photoreversible and photothermal reactions. One can observe that, unlike what is found in thermal kinetics, answers to the latter question do not seem to have been explored in any detail within the literature of photochemistry. A way forwards to shed light on this aspect is to look at the integration of the rate-laws of reactions having a slight incrementation in complexity compared to that of the unimolecular reaction whose product is transparent.