Sesamol (I) was transformed into a fluorescent product (II) and a violet-colored product (IV) by mild oxidation with hydrogen peroxide catalyzed by horseradish peroxidase. The product (II) was elucidated to be 2, 2'-dihydroxy-4, 5 ; 4', 5'-bis (methylenedioxy) biphenyl and the structure of IV was assumed to be the highly conjugated quinoid form of II. The product (II) was transformed into IV by hydrogen peroxide-peroxidase treatment and was regenerated from IV by treatment with reductants. Systems containing II, hydrogen peroxide and peroxidase may provide a new, versatile chromogenic determination method for hydrogen peroxide or peroxidase. Determination of hydrogen peroxide at around 10 μM and peroxidase at around 0.1 nM was possible. Oxidations of I by other means were performed and the results were compared with those obtained with hydrogen peroxide-peroxidase.