From a chemical point of view, oil paintings are not stable objects. Visually, changes in the appearance of an oil painting might be very slow, but over the course of centuries, chemical reactions and physical processes do affect the colour, texture and integrity of oil paint layers. This thesis aims to expand our knowledge of the molecular structure of oil paint and the potential changes to this structure during the life of a painting. Specifically, the issue of metal soap formation—complexes of metal ions and long-chain fatty acids—is a central topic. These complexes can form large crystalline aggregates that protrude through the paint surface, and have been related to cases of brittleness, transparency and delamination in oil paint layers. In Chapters 2 and 3, methods are presented to investigate the molecular structure of metal soaps and their phase behaviour when mixed with liquid or partially polymerised linseed oil. Chapters 4 and 5 contain studies into the different types of zinc carboxylates observed in FTIR spectra of oil paints, and show that certain common pigments leach metal ions that are subsequently bound to carboxylate functionalities on the polymerised oil network. A synthetic model of this ionomerlike state was used to study solvent diffusion and the crystallisation of metal soaps after exposure to fatty acids with time-dependent FTIR spectrometry in Chapter 6. Finally, Chapter 7 presents the progress made towards a complete characterization of the chemical pathways involved in the formation of metal soaps in oil paint.