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 (φΔ) was determined by measurements of the 1O2 luminescence in the near-infrared upon continuous excitation of the sensitizer. Values of φΔ were found to be 0.10 ± 0.02 and 0.14 ± 0.02 in acidic and alkaline media, respectively. Studies of the photo-oxidation 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 (kr) was determined to be 4.9 × 106M−1 s−1 in alkaline medium and the role of 1O2 in the photooxidation of MPT is discussed.