The photochemical behavior of the two isomeric endoperoxides (9,10-PMO2 and 1,4-PMO2) of 1,4-dimethyl-9,10-diphenylanthracene was found to differ in their kinetics and reaction efficiencies. Consistent with the work of Rigaudy et al. and Brauer et al. on a related endoperoxide we find that the generation of 1O2 is wavelength dependent, occurring from upper excited singlet states of the endoperoxide, whereas bond cleavage of the O-O endoperoxide bond occurs principally from the lowest excited singlet and triplet states. Results of picosecond kinetics and absolute quantum yield measurements are discussed in terms of various concerted and nonconcerted mechanisms for the formation of 1O2 and the anthracene fragment. © 1986 American Chemical Society.