Oil paint binding media are complex polymer networks that harbor populations of metal ions. Understanding of the reactivity of these metal ions, often closely linked to paint degradation, is crucial to support paintings conservation efforts. By developing a spectrum fitting approach for the analysis of Fourier transform infrared spectra, we have studied in detail how the molecular structures of zinc carboxylate species in oil paint ionomers change in the lifetime of a painting. It was found that high ZnO pigment content, humidity, and low paint viscosity all stimulate the formation of ionomeric zinc carboxylate species, while the structures (chain or oxo) adopted by those species depend on carboxylate concentration and humidity. Moreover, we found evidence for a difference in reactivity between the two structures for ionomeric zinc carboxylates toward the formation of crystalline zinc soaps. The results have yielded an abundance of information about the internal chemistry of oil paint layers and metal-containing polymers in general.