Thin solid films of C-60 and C-70 have been found to be sensitive to UV-visible light. In the absence of oxygen, which acts as a triplet state quencher, C-60 and C-70 have been observed to phototransform from a toluene-soluble to a toluene-insoluble state. This phototransformation has been studied via Raman and FTIR spectroscopies, UV-visible transmission spectroscopy and laser desorption mass spectroscopy. The results of these experiments have been interpreted as evidence for a phototransformation from a van der Waals solid to one in which the fullerenes are linked by covalent bonds. For C-60, it is proposed that a transformation to a polymeric solid has occurred, whereas a similar flux of UV-visible light applied to C-70 is proposed to lead to a random dimerization of the lattice and a much smaller population of higher oligomers. For both phototransformed C-60 and C-70, the covalent bonds between fullerenes can be broken thermally and the phototransformed material returns to the pristine, toluene-soluble state. UV-visible light can also be used to photochemically assist the diffusion of dioxygen into the interstitial voids in the solid C-60 and C-70 lattices. For C-60, a photochemical enhancement of the O-2 diffusion rate by a factor of similar to 10 is observed by alpha particle backscattering, leading to a stoichiometry of similar to C60O2. Similar to C-60-polyfullerene, C-60(O-2)(x) is also toluene insoluble. As a result, these C-60-based films might find photolithographic applications.