Photophysical properties have been measured and compared for the two known isomers ([6,6]-closed and [5,6]-open) of C61H2, the parent C-60 methano adduct. Optical absorption spectra from the ground and triplet states are consistent with C-60 pi-electron systems that are significantly less perturbed by [5,6] than [6,6] derivatization. However, intrinsic nonradiative decay of triplet state [5,6]-C61H2 is the most rapid yet found in any C-60 adduct, occurring with a characteristic time of 2 mus, a factor of 70 faster than in pristine C-60. By contrast, the triplet decay time of [6,6]-C61H2 is 65 mus under the same conditions. The S, and T, energies of both isomers have been deduced by various methods. It is found that the S-1-T-1 energy gap of [5,6]-C61H2 is larger than that of C-60, whereas the opposite relation holds for [6,6]-C61H2. Quantum yields of triplet state formation and fluorescence were found to be 0.06 and <2 x 10(-5) for the [5,6] isomer and 0.93 and 7.8 x 10(-4) for the [6,6] isomer, respectively. The anomalously low quantum yields for [5,6]-C61H2 indicate efficient nonradiative decay from its S, state with a rate constant estimated at ca.1.5 x 10(10) s(-1). It is concluded that isomeric forms of simple C-60 monoadducts can show major differences in excited-state relaxation.