The photooxidation of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) in thin films and the effects of poly(3-hexylthiophene) (P3HT) on the photooxidation of PCBM were studied using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Two reaction routes of PCBM oxidation and its suppression by P3HT were examined for the first time.