Troglitazone (CS-045) is a new oral antidiabetic drug reported to be effective in insulin-resistant diabetes and to show antihypertensive effects. Photooxidation of troglitazone gave the quinone and quinone epoxide as the major final stable products. An intermediate observed by NMR spectroscopy was shown to be the hydroperoxydienone, which is moderately stable at room temperature. The rate constant of singlet oxygen quenching by troglitazone is 2.14 x 10(8) M(-1) s(-1) and the reaction rate constant in acetone-d6 is 8.64 X 10(6) M(-1) s(-1). Only the chroman ring of troglitazone reacts with and quenches singlet oxygen significantly, and its reactivity and products are analogous to those of alpha-tocopherol. The reactivity of CS-45 toward singlet oxygen is much larger than that of the related compounds lacking the chroman ring.