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Photooxidation and photosensitized oxidation on the formation of volatile compounds in linoleic acid, milk, and lard were studied by a combination of solid-phase microextraction (SPME)-gas chromatography (GC)-mass spectrometry (MS) and headspace oxygen content. Photooxidation is the oxidation under light in the absence of photosensitizers such as chlorophyll and riboflavin. Photosensitized oxidation is the oxidation under light in the presence of photosensitizers. Total volatile compounds in linoleic acid without added chlorophyll under light and in the dark did not increase for 48 hr at 4 °C. Total volatile compounds in linoleic acid with added 5 ppm chlorophyll under light at 4 °C for 0, 6, 12, 24, and 48 hr, increased from 8.9 to 11.6, 21.7, 26.1, 29.3 (1×104) in electronic counts, respectively. 2-Pentylfuran, an undesirable reversion flavor compound in soybean oil, 2-octene-1-ol, 2-heptenal, and 1-octene-3-ol were formed by photosensitized oxidation only. Light excited photosensitizers like chlorophyll can generate singlet oxygen from ordinary triplet oxygen. 2-Pentylfuran, 2-heptenal, and 1-octene-3-ol can come from C10, C12, and C10 hydroperoxide of linoleic acid, respectively, which can be formed b