Under irradiation with visible or UV (>290 nm) light in the presence of molecular oxygen and a minute amount of fullerenes, olefins and dienes undergo ene and Diels-Alder reactions with singlet oxygen to give photooxygenation products. The regio- and stereoselectivities of the photooxygenation of β-myrcene, (+)-pulegone, 4-methylpent-3-en-2-ol, and (+)-limonene were very similar to those observed in known singlet oxygen reactions, indicating that the fullerene-sensitized reaction generates free singlet oxygen. The efficiency of fullerenes and conventional sensitizers was qualitatively examined by using the Diels-Alder reaction between 1O2 and furan-2-carboxylic acid as a probe. Among those examined, C70 was found to be the most effective. The reaction was the fastest and completed with as little as 0.0001 equiv of C70. C60 and hematoporphyrin were found to be of similar efficiency. The methanofullerene 13, which lacks one olefinic conjugation in the C60 core, was as good as C60 itself, but the aminofullerene 14, lacking six double bonds, was quite inferior. The fullerene carboxylic acid 15, which was previously shown to show considerable biochemical activity, was found to be capable of generating singlet oxygen in aqueous DMSO.