Pterins are a family of heterocyclic compounds present in a wide range of living systems that participate in relevant biological functions and are involved in different photobiological processes. 6-Methylpterin (MPT) was investigated for its efficiency of singlet-oxygen (¹O₂) production and quenching in aqueous solution. The quantum yields of ¹O₂ production (Φ_Δ) was determined by measurements of the ¹O₂ luminescence in the near-infrared upon continuous excitation of the sensitizer. Values of Φ_Δ were found to be 0.10 ± 0.02 and 0.14 ± 0.02 in acidic and alkaline media, respectively. Studies of the photooxidation of MPT in acidic (pH = 5.0–6.0) and alkaline (pH = 10.2–10.8) aqueous solutions at 350 nm and room temperature have been performed. The photochemical reactions were followed by UV-visible spectrophotometry, high-performance liquid chromatography and an enzymatic method for H₂O₂ determination. MPT is not light sensitive in the absence of oxygen, but it undergoes a photooxidation reaction in the presence of oxygen, yielding several nonpteridinic products. The quantum yields of MPT disappearance were determined and values of 2.4 (±0.5) × 10⁻⁴ and 8.1 (±0.8) × 10⁻⁴ were obtained in acidic and alkaline media, respectively. H₂O₂ was detected and quantified in irradiated solutions of MPT. The rate constant of the chemical reaction between ¹O₂ and MPT (k_r) was determined to be 4.9 × 10⁶ M⁻¹ s⁻¹ in alkaline medium and the role of ¹O₂ in the photooxidation of MPT is discussed.