Derivatives of riboflavin, specifically flavin mononucleotide, have recently been in the spotlight as the active component in protein-encased optogenetic sensitizers to produce singlet oxygen, O₂(a¹Δ_g). The extent to which riboflavin deactivates singlet oxygen is a key aspect of these studies. Surprisingly, there is a dearth of information on the rate constant for riboflavin-mediated singlet oxygen removal. We now report that, when riboflavin is dissolved in both aqueous and methanol solutions, the rate constant for riboflavin-mediated removal of singlet oxygen cannot be greater than ∼ 2 × 10⁵ s⁻¹ M⁻¹. This number is appreciably smaller than what has been reported in the literature and, as such, our results can be significant for mechanistic interpretations of photo-processes that involve flavins in oxygenated systems.