Photophysical and photochemical properties of C-120, the [2+2]-dimer of C-60, have been investigated. In toluene solution, the 327 nm ground-state absorption peak has a molar absorptivity of 113 000 dm(3) mol(-1) cm(-1), somewhat more than twice that of the analogous 336 nm peak of C-60. The S-1 and T-1 origin energies in C-120 are found to be lower than those in C-60 by approximately 1070 and 625 cm(-1), respectively. The T-1 state's intrinsic exponential lifetime at 296 K is 42 +/- 2 mus, or a factor of 3.4 times shorter than for C-60. The T-n <-- T-1 absorption spectrum of C-120 shows bands at 710 and 1060 nm. The former is similar to those seen in [6,6]-adducts of C-60, but the latter may reflect electronic coupling between the two halves of the dimer molecule. In degassed room-temperature toluene solution, C-120 photodissociates to C-60 with a quantum yield of 2.3 x 10(-3). This dissociation occurs from the T-1 state and is strongly suppressed by dissolved oxygen. The rate constant for T-1 dissociation increases from ca. 55 s(-1) at 297 K to 800 s(-1) at 333 K, implying an Arrhenius activation energy near 64 kJ mol(-1) (5360 cm(-1)) and a prefactor in the range of vibrational frequencies.