Using Positronium (Ps) atom as a fundamental probe that maps changes in the local electron density of the microenvironment and high resolution transmission electron microscopy, C60 aggregation in neat CS2 solvent is reported over a concentration range 0.02to2.16 g/dm3. Spontaneous formation of stable spherical C60 aggregates in the colloidal range (∼90–125 nm) was observed over a critical concentration range of 0.06–0.36 g/dm3, beyond which the clusters broke. Specific interactions of the Ps atom with the surrounding revealed the onset concentration for stable aggregate formation in this solvent to be 0.06 g/dm3. The solution phase C60 structure in the critical concentration range was analyzed to be a spherical fractal aggregate with a fractal dimension of 1.9 and the growth mode followed a diffusion limited cluster aggregation mechanism. At concentrations beyond 0.36 g/dm3, an entropy driven phase change was noticed leading to the formation of irregular, but oriented crystalline components. A microscopic diffusion model was applied to calculate the o-Ps lifetime density function and diffusion coefficients of o-Ps and the C60 aggregates in the solution. With randomly distributed C60 fractal clusters, the o-Ps density function resulted in a good agreement between the calculated and the experimental o-Ps lifetimes, revealing the diffusion coefficients of C60 fractal cluster and the o-Ps to be 2.27×10−6 cm2/s and 25.1×10−5 cm2/s respectively. © 2003 American Institute of Physics.