The effects of 0, 5, 10, and 20 ppm β-carotene on the oxidation of 0.033, 0.066, 0.165, 0.330, or 0.660M soybean oil in methylene chloride containing 4 ppm chlorophyll have been studied by measuring headspace oxygen depletion in gas-tight sample bottles during 24 hr of light storage. The oxygen depletion was measured by a thermal conductivity gas chromatograph. The rate of singlet oxygen formation in soybean oil by 4 ppm chlorophyll was 0.17 μmoles O2/mL head-space during first 24 hr of storage. The quenching mechanism study of g-carotene on the chlorophyll sensitized photooxidation of soybean oil showed that g-carotene minimized the oxidation of soybean oil by quenching singlet oxygen. p-carotene did not quench chlorophy11.