This report introduces a Monte Carlo simulation method to quantitatively deconvolute the size-dependent UV-vis absorption spectra of colloidal semiconductor nanocrystals. The method was experimentally verified and then applied for studying the transition stage from net dissolution to Ostwald ripening of CdS nanocrystals in a few nanometers size range quantitatively using the UV-vis spectra recorded in situ. Interparticle interaction in such processes was identified, which strongly affected the temporal evolution of the size and size distribution in a system. A preliminary model was proposed to quantitatively account for these observations by considering both solution-particle and particle-particle diffusion. The results indicate that crystallization processes of nanocrystals are substantially different from that of large-sized crystals.