The photomediated synthesis of silver right triangular bipyramids and prisms has been studied, and we have determined that pH and [BSPP]/[Ag+] ratio (bis(p-sulfonatophenyl)phenylphosphine, BSPP) finely control the reaction rate and, consequently, the crystal growth pathway and morphology of final products. A fast reaction rate, realized at a high pH such as 10 or 11 and a [BSPP]/[Ag+] ratio close to 1.0, is necessary to synthesize (100)-faceted right triangular bipyramids in high yield by preferential deposition on (111) facets of planar-twinned seeds. A slower reaction rate, which occurs at lower pH or higher [BSPP]/ [Ag+] ratios, results in preferential deposition on (100) facets of planar-twinned seeds and the formation of nanoparticles possessing a larger surface area defined by (111) facets, such as truncated triangular bipyramids or prisms. BSPP further influences the reaction rate by ensuring a relatively constant concentration of aqueous Ag+. In the absence of BSPP, the aqueous [Ag+] steadily decreases as it is consumed and results in a continuously decreasing reaction rate, which changes the preferred facet for silver deposition. At the beginning of the reaction, growth on (111) facets almost exclusively occurs and results in the formation of right triangular bipyramids, which only have (100) facets. When the reaction rate is decreased due to the consumption of Ag+ through the course of the reaction, the facet deposition preference changes from (111) to (100) and results in the formation of truncated bitetrahedra, with (111) facets, as the predominant product.