The origin of the optical nonlinearity of an organic polymeric composition based on hole-conducting poly(9-vinylcarbazole) and fullerene C 70 was investigated by two-beam coupling, self-action and z-scan of a cw laser beam at 633 nm, and spectroscopically. The large value of the nonlinearity of the composite was explained by the difference in polarizabilities of excited and unexcited fullerenes C 70 and its light-induced anion-radicals C 70 −. The complicated nonlinear dynamics is caused by population changes of electron levels of the fullerene molecule depending on the hole migration and its trapping or recombination.