(1) A method has been developed for studying the photo-oxidation of plant pigments on thallous bromide; it depends essentially on the accurate measurement of the pressure decreases involved, using a sensitive Bourdon glass-spring gauge. (2) The results show that, under these conditions, both freshly prepared and copper-stabilised chlorophyll absorb one molecule of oxygen par molecule of pigment. This is also true for the carotenoid fraction. For acid-treated chlorophyll, or phebphytin, this ratio is increased threefold. These oxidations were shown to be partly reversible. (3) The thallous bromide has been shown to act merely as a photo-sensitiser. (4) A thermal oxidation of carotenoids and of carotenoid/chlorophyll mixtures was observed in the presence of certain cobalt salts. (5) It has proved Impossible as yet, with this in vitro system, to simulate any photosynthetic reaction, although the effect of carbon dioxide and water vapour was examined for the majority of the experiments. (6) A general scheme has been proposed to account for the main features of the photo-oxidation.