Irradiation of 9,10-dichloroanthracene (DCA) in the presence of 2,5-dimethyl-2,4-hexadiene (DMHD) in benzene at 25 degrees C gives a single adduct (Ad) corresponding to [4 + 2] addition of DMHD to the 9,10 positions of DCA. Quantum yields for Ad formation are reported as a function of [DMHD], [DCA], and methyl iodide concentration, [MeI]. Fluorescence quantum yields and lifetimes of DCA and (DCA/DMHD) singlet exciplex as a function of [Mel] are also reported. Transient absorption measurements reveal the time evolution of excited DCA singlet, (1)DCA*, and triplet, (3)DCA*, states and of the (DCA/DMHD) singlet exciplex, (1)(DCA . DMHD)*. They show that the dominant decay path from (1)(DCA . DMHD)* gives (3)DCA*. Quenching of (1)DCA* and exciplex fluorescence and transient absorption by Mel are observed. The dependence of Ad quantum yields on [DMHD] and the strict proportionality between product quantum yields and exciplex fluorescence quantum yields at different [MeI] establish a singlet mechanism for adduct formation. Assuming that Ad is a primary photoproduct, the results are consistent with Yang’s proposal of stepwise collapse of polar exciplexes to seemingly forbidden cycloadducts. A singlet biradical could be the intermediate, but intersystem crossing of the singlet exciplex to a triplet biradical as the initial step cannot be ruled out. The loss of DCA is enhanced at very high [DMHD] but oxygen quenching experiments show that this enhancement does not involve formation of the triplet biradical by addition of (3)DCA* to DMHD.