Consequent to our recent papers on C-60 colloidal aggregates in CS2 solution (Bokare, A. D.; Patnaik, A. J. Phys. Chem. 2003, 107, 6079-6086) and their probable electron density distribution (Bokare, A. D.; Patnaik, A. J. Chem. Phys. 2003, 119, 4529-4538), the solution-phase structure of C-60 in nonpolar o-xylene is reported using a positronium (Ps) atom as a fundamental probe, mapping changes in the local electron density of the microenvironment. Spontaneous formation of stable aggregates in the colloidal range (similar to90-150 nm) was observed in a concentration range of 0.14-0.36 g/dm(3), beyond which they broke. An onset concentration for as against 0.06 g/dm(3) for the polar CS, solvent, was noted and was aggregate formation at 0.14 g/dm(3) substantiated by complete quenching of pyrene fluorescence at and beyond this onset due to photoinduced electron transfer from the pyrene excited state to the C-60 aggregate. An order-disorder phase transition led to a notable geometry change of the colloidal particles; a sphere-to-nonuniform cylinder transition following an increase in the C-60 concentration from 0.14 to 0.36 g/dm(3) revealed the aggregate curvature/internal modes to have been influenced by energetic/entropic and/or hydrodynamic interactions in the solvent medium. Transmission electron microscopy images of the aggregated clusters, in corroboration with Ps annihilation characteristics and pyrene fluorescence, revealed the clusters to be hexagonally close packed microcrystals.