Oil paints have been used for hundreds of years in the European tradition of panel and canvas painting. Scores of different formulas have been used to find the correct paint with the desired properties. The alteration of the simple oil and pigment formula has led to elaborate systems with complex aging and solvent behavior. In the early stages of paint aging, the processes of autoxidation and hydrolysis dominate and determine overall structure, whereas hydrolysis determines long-term structural behavior. The ions and small molecules set loose by these processes further complicate the solvent behavior. Early hydrolysis of oils can lead to free fatty acids that can alter the initial chemistry and mechanical properties of paint films. The diffusion of small molecular weight compounds through the films of a painting may be quite extensive, with metal ions influencing drying rates and fatty acid anions (or neutral compounds) affecting stiffness or flexibility as well as forming accretions. Removal of these small compounds by solvents can result in loss of color saturation, as well as in embrittling the paint film by promoting cracking, thus affecting its overall strength and leading to an eventual collapse of the paint structure.