Copper nanomaterials are being used in a large number of commercial products because these materials exhibit unique optical, magnetic, and electronic properties. Metallic copper nanoparticles, which often have a thin surface oxide layer, can age in the ambient environment and become even more oxidized over time. These aged nanoparticles will then have different properties compared to the original nanoparticles. In this study, we have characterized three different types of copper-based nanoparticle (NP) samples designated as Cu(new) NPs, Cu(aged) NPs, and CuO NPs that differ in the level of oxidation. The solution phase behavior of these three copper-based nanoparticle samples is investigated as a function of pH and in the presence and absence of two common, complexing organic acids, citric and oxalic acid. The behavior of these three copper-based NP types shows interesting differences. In particular, Cu(aged) NPs exhibit unique chemistry including oxide phases that form and surface adsorption properties. Overall, the current study provides some insights into the impacts of nanoparticle aging and how the physicochemical characteristics and reactivity of nanomaterials can change upon aging.