Significance Singlet fission is a process in which one of two adjacent organic molecules absorbs a single photon, resulting in rapid formation of one triplet state on each of the molecules. Theory predicts that if the two triplet states can separate and then generate two electron–hole pairs, the efficiency of a solar cell employing this material could rise from an upper limit of about 33 to 45%. Electron spin dynamics of the two triplet states can produce singlet, triplet, or quintet states having zero, two, or four unpaired electrons, respectively. We have discovered that triplet–quintet-state mixing at room temperature can open a pathway that annihilates triplet states, potentially diminishing the performance of solar cells using singlet fission.