The semi-empirical method can, in principle, apply to any photo- or photothermal reaction. So far, it has produced equations for the kinetics of five photoreactions, namely, the unimolecular reaction, XY(1Φ) with \( {\varepsilon}_Y^{\lambda_{n- isos}}\ne 0 \) ; the reversible XY(2Φ), XY(1Φ, 1k), and XY(2Φ, 1k) reactions (where the thermal reaction-step is Y → X); and the multi-consecutive reaction involving four photoproducts and four photochemical reaction-steps, XY4(4Φ). The order in which these reactions are listed follows, in fact, the chronology that should be adopted for such studies, adhering to a progressive incrementation of the complexity of the mechanism. This strategy is justified by the novelty of SEM, which imposes that each new reaction has to be individually checked, the details of which are presented hereafter.