We report observations of an enhancement of at least 3 orders of magnitude in the rate for photoinduced cross linking of C-60 films under an ambient NO atmosphere. Under extended illumination (for as long as 48 h at 8 W/cm(2)) the fullerene clusters disappear, and the film evolves toward an amorphous mixture of C, N, and O. Raman and XPS data show signatures of C-N bond formation, We propose a mechanism for the role of NO based on stabilization of the diradical intermediate formed in the primary photochemical step. This mechanism suggests a new generic pathway for photoinduced transformation of fullerene films. Related experiments with samples removed from NO and exposed to air also show a residual polymerization effect with a decay time on the order of an hour.