Ligand exchange reactions are commonly used to alter the surface chemistry of metal chalcogenide quantum dots; however, a lack of quantifiable data for these processes limits the rational functionalization of nanomaterials. Here, we quantify the X-type ligand exchange reaction between carboxylate-terminated ligands on PbS quantum dots via1H NMR spectroscopy. Using spectroscopic handles of both the native and exchange ligand, bound and free forms of each have been quantified as a function of exchange ligand concentration. We find that the equilibrium constants for the reaction between oleate-capped PbS quantum dots and undec-10-enoic acid are 2.23 ± 0.50 and 2.14 ± 0.42 for sets of nanocrystals prepared by two different synthetic methods. X-ray photoelectron, absorbance, and emission spectroscopies indicate that the carboxylate exchange reaction does not alter the lead ion coverage of the nanocrystal surface. The quantitative equilibrium constant determined herein can be used to improve control over partial ligand exchange reactions on PbS nanocrystals.