The mechanism of formation of rhodium nanocubes in the presence of tetradecyltrimethylammonium bromide and polyvinylpyrrolidone in ethylene glycol was studied in situ by XAFS, in conjunction with other ex situ techniques including MALDI-TOF MS, XRD, TEM, and UV-vis spectroscopy. Detailed analysis of the XAFS data reveals that Rh nanocubes are formed in four distinct stages comprising ligand substitution., slow nucleation, continued nucleation/fast growth of nuclei, and shape correction, which were corroborated by the other techniques. All the main types of Rh species and the chemical transformations taking place at each stage were clearly identified. In particular, combined XAFS and MAILD-TOF MS techniques provide compelling evidence that a Rh4 cluster acts as the critical nuclei species for the formation of the nanocubes. Other key findings include (1) RhBr3 is the true precursor of the Rh nanocubes, (2) the consumption rate of the precursor appears to follow Finke-Watzky type kinetics, and (3) non-cubic-shaped nanoparticles can be transformed into nanocubes, even if all the precursor has been exhaust. These results provide a clear understanding of the formation mechanism of the rhodium nanocubes, which is essential for rational design of nanoparticles.