Phytofluene ex tomatoes is subject to cis-trans isomerization. The native form in the tomatoes studied is a relatively thermostable but photosensitive cis compound that may rearrange spontaneously or when illuminated or under the catalytic influence of iodine in light. It then yields the more stable all-trans form possessing stronger adsorbability. The two main stereoisomers mentioned were characterized by spectroscopic and chromatographic methods. ; © 1952 American Chemical Society. Received June 19, 1951.