The mechanism of the two-phase Brust-Schiffrin synthesis of alkane thiol protected metal nanoparticles is known to be highly sensitive to the precursor species and reactant conditions. In this work X-ray absorption spectroscopy is used in conjunction with liquid/liquid electrochemistry to highlight the significance of Brˉ in the reaction mechanism. The species [AuBr4]ˉ is shown to be a preferable precursor in the Brust-Schiffrin method as it is more resistant to the formation of Au(I) thiolate species than [AuCl4]ˉ. Previous literature has demonstrated that avoidance of the Au(I) thiolate is critical to achieving a good yield of nanoparticles, as [Au(I)X2]ˉ species are more readily reduced by NaBH4. We propose that the observed behavior of [AuBr4]ˉ species described herein explains the discrepancies in reported behavior present in the literature to date. This new mechanistic understanding should enable nanoparticle synthesis with a higher yield and reduce particle size polydispersity.