Nucleation has been generally acknowledged as a rapid but uncontrollable process that is difficult to decouple from the subsequent growth phase. Here, we report our finding that nucleation of semiconductor magic-size clusters (MSCs) can be well-regulated, without a subsequent evolution in size. Colloidal semiconductor CdS MSCs were synthesized by a two-step approach intentionally designed, without the simultaneous formation of nanocrystals of other sizes. The nuclei MSCs exhibit a sharp optical absorption peaking at 311 nm and are thus denoted by MSC–311. We prepared the immediate precursor for MSC–311 denoted by IP311 which is liquid-like, through a reaction which was normally performed to grow CdS conventional quantum dots (QDs), but at a different temperature (180 °C) prior to the nucleation and growth of CdS QDs. We demonstrate that the nucleation of MSC–311 from IP311 followed first order kinetics remarkably well, and the presence of a small amount of methanol accelerated this process effectively. Moreover, the liquid-like prenucleation cluster IP311 and the nuclei MSC–311 have similar masses. Accordingly, we propose that the intramolecular reorganization of IP311 results in the nuclei MSC–311, the formation of which features a two-step nucleation pathway. The present study introduces methodology via absorption spectroscopy to monitor the nucleation kinetics of semiconductor MSCs from their immediate precursors. The repeatable, predictable, and controllable nucleation process investigated here brings a deeper insight into nucleation of other semiconductor nanocrystals and contributes to the foundation for the future development of advanced theoretical models for crystal nucleation.