The frequency domain technique was applied to measure the effect of n-propyl gallate (nPG) on the apparent photodegradation rate of fluorescein in aqueous solutions. The illuminating light was modulated and the change in fluorescence from the illuminated region was detected synchronously. A constant flow rate was imposed on the fluorescein solution to control the mass transport of fluorescein into the illuminated region. The photodegradation response was described by a model which assumed two steps: (1) singlet oxygen production via energy transfer between the excited triplet state of fluorescein and molecular oxygen in the ground triplet state and (2) photodegradation via the interaction of fluorescein with singlet oxygen. It was assumed that nPG affects the photodegradation of fluorescein by quenching the oxygen singlet state. In the context of this model, the rate of singlet oxygen quenching by nPG was found to be (1.3 +/- 0.2) x 10(9) s(-1)M(-1). The product of the singlet oxygen photosensitization rate, k(ox), and the photodegradation rate, k(pd), was k(ox)k(pd) = (0.60 +/- 0.3) x 10(17) s(-2) M-2. Photodegradation was observed in argon purged solutions and high concentrations of nPG, suggesting another photodegradation mechanism, such as direct electron transfer between fluorescein in the excited triplet state and fluorescein in the ground state. nPG also quenches the excited singlet state of fluorescein with a rate of (2.5 +/- 0.3) x 10(9) s(-1) M-1.