Photo-induced structural transformations of C-60 single crystals under pressures up to 30 GPa were investigated by means of optical transmittance and photoluminescence. We found that the band gap energy shifts toward lower energies at a rate of -16 meV/GPa with an increase in the pressure applied during photo-irradiation. The activation energy of the photo-induced structural transformation at 3 GPa was estimated to be 0.23 eV. Analysis of the optical spectra estimates the number of [2 + 2] cycloadditional bonds created by photo-irradiation under pressure, and indicates that the intermolecular bonds expand to three dimensions to sustain highly contracted structure. (C) 2003 Elsevier B.V. All rights reserved.