We report an approach based on a combination of inductively coupled plasma mass spectrometry and X-ray photoelectron spectroscopy for quantitative analysis of the role played by Br- ions in the synthesis of Pd nanocrystals. The Br- ions were found to adsorb onto Pd{100} facets selectively with a coverage density of ca. 0.8 ion per surface Pd atom. The chemisorbed Br- ions could be removed via desorption at an elevated temperature under reductive conditions. They could also be gradually released from the surface when Pd cubic seeds grew into cuboctahedrons and then octahedrons. On the basis of the coverage density information, we were able to estimate the minimum concentration of Br- ions needed for the formation of Pd nanocubes with a specific size. If the concentration of Br- ions was below this minimum value, not all of the {100} facets could be stabilized by the capping agent, leading to the formation of nanocubes with truncated corners. The quantitative analysis developed in this study is potentially extendable to other systems involving chemisorbed capping agents.