The description of photokinetics under isosbestic irradiation (at λisos), detailed in Part III, has advanced our understanding of the photothermal reaction’s kinetic behaviour and characteristics but did not inform about the features of the reaction when the employed monochromatic light is non-isosbestic (at λn − isos). Our investigation of photokinetics under this type of irradiation light would naturally start from the basic photoreaction, the simplest that occurs in the realm of photochemical transformations, that is, the primary photoprocess, X → Y. This reaction is also considered because despite its simplicity, its kinetics has not been described in the literature at the required level of detail (i.e. when the monochromatic irradiation light is non-isosbestic). It turns out that unravelling and understanding the properties of this primary photoprocess under λn − isos-irradiation shapes a template that will allow setting up the fundamental bases for a larger photokinetics endeavour (as shall be seen in Chaps. 14 and 19 ).