Hypothesis: Ligand exchange is a widely-used method of controlling the surface chemistry of nanomaterials. Exchange is dependent on many factors including the age of the core particle being modified. Aging of the particles can impact surface structure and composition, which in turn can affect ligand binding. Experiments: To quantify the effects of aging on ligand exchange, we employed a technique to track the exchange of radiolabeled C-14-oleic acid with unlabeled, oleic acid bound to iron oxide nanoparticles. Liquid scintillation counting (LSC) was used to determine the amount of C-14-oleic acid adsorbing to the particles throughout the duration of the exchange for particles aged for 2 days, 7 days, and 30 days. Findings: Results revealed an increase in the total amount of ligands exchanged with aging up to 30 days. Kinetic analysis of these results revealed a significant decrease in the overall rate of ligand exchange between 2 and 30 days. The change in extent of adsorption with age could suggest increased availability of free binding sites. A follow-up study comparing exchange with oxidized and unoxidized particles suggested this increase in ligand adsorption may be due to changes in the Fe2+/Fe3+ ratio on the surface as the particles aged. (C) 2017 Elsevier Inc. All rights reserved.