We demonstrate that stable radicals, such as molecular oxygen and TEMPO, can inhibit back electron transfer by enhancing intersystem crossing of a singlet radical ion pair into its triplet state. The phenomenon is demonstrated on a series of dyads between porphyrin/chlorin and fullerene C-60. The effect is observed only when the energy of the charge separated state is lower than that of the locally excited triplet states. Because of the spin statistics, the reverse intersystem crossing is less efficient, allowing use of oxygen and other paramagnetic species for impeding charge recombination in various electron-transfer systems. The interpretation is also confirmed by measurements of singlet oxygen yield and (lack of) magnetic field effects.