Lack of detailed understanding of the growth mechanism of CsPbBr 3 nanocrystals has hindered sophisticated morphological and chemical control of this important emerging optoelectronic material. Here, we have elucidated the growth mechanism by slowing the reaction kinetics. When 1bromohexane is used as an alternative halide source, bromide is slowly released into the reaction mixture, extending the reaction time from ∼3 s to greater than 20 min. This enables us to monitor the phase evolution of products over the course of reaction, revealing that CsBr is the initial species formed, followed by Cs 4 PbBr 6 , and finally CsPbBr 3 . Furthermore, formation of monodisperse CsBr nanocrystals is demonstrated in a bromide-deficient and lead-abundant solution. The CsBr can be transformed only into CsPbBr 3 nanocubes if additional bromide is added. Our results indicate a fundamentally different growth mechanism for CsPbBr 3 in comparison with more established semiconductor nanocrystal systems and reveal the critical role of the chemical availability of bromide for the growth reactions.