The sensitized photooxidation promoted by daylight-absorbing compounds appears as a plausible course to produce the photodegradation of catecholamines. We report the kinetics and mechanism of vitamin B₂ (riboflavin [Rf])–sensitized photooxidation of isoproterenol (Iso), a synthetic sympathomimetic drug structurally related to epinephrine, using water as a solvent. A weak dark complex Rf–Iso is formed, only detectable at relatively high Iso concentrations (>10 mM), with a mean value of 13 ± 3 M⁻¹ for the apparent association constant. Under aerobic sensitizing conditions (Rf ∼ 0.02 mM and Iso ∼ 0.5 mM) two oxidative mechanisms operate, mediated by singlet molecular oxygen (O₂(¹Δ_g)) and superoxide radical anion (O₂^·−). Our analysis shows that the main reaction pathway is an electron transfer–mediated quenching of Rf excited triplet state (³Rf^*) by Iso. It produces the species Iso^· and Rf^·−. The latter, in a subsequent reaction path, generates O₂^·−, which is mainly responsible for Iso photooxygenation. In a less-important process, energy transfer of the ³Rf^* to dissolved oxygen generates O₂(¹Δ_g). The kinetic balance between chemical and physical quenching of O₂(¹Δ_g) by Iso indicates that the process is largely dominated by the physical, not chemical, interaction. The results, which can be extrapolated to an in vivo condition, show the susceptibility of Iso to undergo visible light–induced photodegradation in the presence of dye sensitizers present in the environment.