Doxorubicin exhibits unusual photoreactivity in aqueous solutions. Our data show that there are two distinct photoreactive pathways for doxorubicin. One is a two-step process that leads to the formation of 3-methoxysalicylic acid, a stable degradation product. The other pathway is a photoreduction of doxorubicin to form the corresponding dihydroquinone, which undergoes spontaneous oxidation mediated by dissolved oxygen to recover doxorubicin with the formation of hydrogen peroxide. Our data account for the known nonlinear dependence of doxorubicin fluorescence intensity on concentration.