Photo-retro-Diels-Alder (PrDA) reactions of a variety of Diels-Alder (DA) adducts were studied. Experimental results showed that the photoreactivity (quantum yield) depends on the electron-donating ability of the cliene component and the electron-withdrawing ability of the dienophile component. The mechanism was studied by trapping the reaction intermediate, O(2) quenching, time-resolved absorption, and fluorescence spectroscopy. All the results support a mechanism that involves a charge-separated intermediate generated from a singlet excited state. The PrDA reaction may find applications in photoresponsive materials, photolithography, drug delivery, and mechanistic research.