The ability of fucoxantine for photooxidation by oxygen in a number of organic solvents and for photoreduction by oxorbic acid and phenylhydrazine in pyridine is demonstrated. The reactions are irreversible under common conditions. At the same time fucoxanthine possesses a pronounced ability for photosensitization of red-ox reaction between methyl red and ascorbic acid or phenylhydrazine, which suggests a formation of reversible very labile intermediate pigment compounds at initial steps of photoreaction.