The photochemistry and the irreversible photodecomposition of naphthacenequinones have been studied. Picosecond, nanosecond, and longer lived transient spectra have been recorded and the kinetics of the intermediates measured and assigned to specific transient species. On the basis of the data, a mechanism for the photoinduced structural rearrangement is proposed. The adiabatic reaction is found to proceed via triplet states and the irreversible decomposition due to excited triplet abstraction of hydrogen from the solvent.