Halides are often used in the shape-controlled synthesis of noble metal nanostructures, in particular, chloride in the form of HCl is frequently added to the polyol synthesis of Ag nanocubes (NCs). We demonstrated the shape of Ag nanostructures depend on the quantity, not the form of Cl- added to the synthesis. Low Cl- concentrations led to cuboctahedron, while higher Cl- concentrations yielded cubes. Free Cl- or Cl- in the form of AgCl(s) both led to NC formation during a polyol synthesis. In this work, the authors hypothesized controlled Cl- release (i.e., AgCl dissolution) must be commensurate in rate with (auto)catalytic reduction of Ag+ to achieve Ag NCs during the polyol reduction. This hypothesis is based on observations that AgCl cubes form immediately during the polyol synthesis and are absent at the end of a successful Ag NC synthesis. The direct replacement of HCl with HBr in the polyol synthesis leads to the formation of truncated octahedra, terminated by {111} facets. However, conducting the synthesis with a combination of HCl and HBr produces nanorods with high yield. Time-resolved XRD revealed the in situ formation of mixed halide salts (AgClxBr1-x) during synthesis. The Ksp of AgCl is several orders of magnitude larger than that of AgBr, with the ternary crystal structures expected to possess intermediate solubilities. We synthesized uniform AgClxBr1-x nanoparticles ex situ, which allows us to finely control the Cl:Br ratio, and therefore the release of Cl- due to the variable solubility of the mixed halide salts. A range of dissolution kinetics can be achieved by altering the Cl:Br ratio. However, the addition of free Br- has found little success in the Ag NC synthesis. We hypothesize HBr reacts with the polyol solvent to produce water, which then reduces the PVP end groups from aldehyde to hydroxyl in acidic conditions which are known to produce multiply twinned seeds. Therefore, we utilized perturbative experiments which take advantage of the temporally distinction between nucleation and growth to isolate the effect of Br- on Ag halide dissolution kinetics during NC formation. For further evidence, we have employed AgClxI1-x nanocrystals in Ag NC syntheses, which require much less I- than Br- to have an appreciable impact on solubility, since the Ksp for AgI is orders of magnitude lower than AgBr. These approaches demonstrate commensurate rates between dissolution and silver reduction are required to produce monodispersed Ag NCs.