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, C-60 aggregation in neat CS2 solvent is reported over a concentration range 0.02 to 2.16 g/dm(3). Spontaneous formation of stable spherical C-60 aggregates in the colloidal range (similar to90-125 nm) was observed over a critical concentration range of 0.06-0.36 g/dm(3), 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/dm(3). The solution phase C-60 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/dm(3), 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 C-60 aggregates in the solution. With randomly distributed C-60 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 C-60 fractal cluster and the o-Ps to be 2.27x10(-6) cm(2)/s and 25.1x10(-5) cm(2)/s respectively. (C) 2003 American Institute of Physics.