C-60 aggregation in neat CS2 solvent is reported using positronium (Ps) as a fundamental probe, which maps changes in the local electron density of the microenvironment. Specific interactions of the Ps atom with the surrounding dramatically reveal the onset concentration for stable aggregate formation in this solvent to be 0.06 g/dm(3). Spontaneous formation of stable aggregates in the colloidal range (similar to90-125 nm) was observed over a narrow concentration range of 0.06-0.36 g/dm(3), beyond which the clusters broke. Variation of aggregate properties with size (proportional to tau(3) [PS lifetime] and/or lambda(3) [Ps annihilation rate, 1/tau(3)]) are noteworthy. Ps annihilation characteristics in corroboration with transmission electron microscopy and UV-vis absorption spectroscopy demonstrate the solution phase C-60 structure to be a spherical fractal aggregate with fractal dimension 1.9 and that the growth mode follows a diffusion-limited cluster aggregation mechanism. At a higher concentration beyond 0.36 g/dm(3), an entropy driven phase change was noticed leading to formation of irregular, but oriented crystalline components.