The photolysis half-lives of 70 polychlorinated dibenzo-p-dioxins and dibenzofurans are correlated with their molecular structures by a QSPR model (R-2 = 0.72) comprising three bond-energy-related descriptors. The photodegradation depends on the stability of the aromatic system and the C-O and C-C bond strengths. Model validation utilized leave-one-out (R-2 = 0.69), leave-many-out (R-2 = 0.72), and scrambling (R-2 = 0.19) procedures. Our results allow estimation of the photolysis half-lives of the remaining possible 140 PCDDs and PCDFs congeners.