An in-depth study of the intramolecular [5 + 2] photocycloaddition of maleimides using tunable UVlasers has demonstrated that the peak in quantum yield and rate both occur at wavelengths some 50 nmred shifted from the maxima observed in the UV spectra. A detailed explanation for these findings usingtime-dependent DFT calculations is presented, and the implications for a previously adopted mechanismare discussed.