In this work, we present a combined experimental and theoretical analysis of two-component ligand shells passivating CdSe quantum dots. Using nuclear magnetic resonance spectroscopy, we first show that exposing oleatecapped quantum dots to primary carboxylic acids results in a one-for-one exchange that preserves the overall ligand surface concentration. Exposure to straight-chain acids leads to a binary ligand shell that behaves as an ideal mixture and that has a composition matching the overall acid composition of the dispersion. In the case of branched-chain acids, the exchange is restricted to about 25% of the original ligands. Based on molecular dynamics simulations, we argue that this behavior reflects the more favorable packing of oleates compared to "branched carboxylates on the (100) facets of CdSe quantum dots.