Pharmaceutical and personal care products (PPCPs) are a group of emerging contaminants that have frequently been detected in aqueous environments. Phototransformation driven by solar irradiation is one of the most important natural processes for the elimination of PPCPs. In this study, primidone (PMD) was chosen as a model “photorefractory” compound. A series of experiments were conducted to assess if reactive intermediates (RIs), such as hydroxyl radical (HOradical dot), singlet oxygen (¹O₂), and triplet states of dissolved organic matter (³DOM^⁎), inhibited or enhanced the photochemical transformation of PMD under simulated solar irradiation. The results indicate that HOradical dot plays a key role in the photodegradation of PMD and that dissolved oxygen can affect the degradation rate of PMD by promoting HOradical dot formation. Our results demonstrated that PMD can not only react with free HOradical dot (HOradical dot_-free) but also react with lower-energy hydroxylation agents (HOradical dot_-like). The contributions of HOradical dot_-free and HOradical dot_-like to PMD degradation in various dissolved organic matter (DOM) solutions were estimated by a methane-quenching experiment. The results indicated that the HOradical dot_-like species were important in the photodegradation of “photorefractory” compounds. The bimolecular reaction rate constant of the reaction of free HOradical dot with PMD was measured as (5.21 ± 0.02) × 10⁹ M⁻¹ s⁻¹ by using electron pulse radiolysis. Furthermore, PMD was used as a probe to estimate the steady-state concentration of HOradical dot_-free in various DOM solutions. Using the multivariate statistical strategies of orthogonal projection to latent structures discriminant analysis (OPLS-DA) and hierarchical clustering, 28 photochemical transformation products (TPs) of PMD were successfully identified from the DOM matrix.