A singly bonded fullerene dimer [C60R](2) in the solid state thermally generates a pair of fullerene radicals C60R center dot that dissociate reversibly and irreversibly upon heating and cooling of the solid. The temperature dependence of the electron mobility of the solid shows striking nonlinearity, caused by the dissociation of a strongly interacting radical pair into two free radicals, which interact with the neighboring fullerene molecules to increase the mobility 10 times to a value of 1.5 X 10(-3) cm(2) V-1 s(-1). The nonlinearity is due to the plastic crystalline nature of fullerene crystals.