Although oxalate salts have been the subject of extensive research as alteration products on calcareous substrates (e.g. stone and fresco), there has been relatively little discussion of their occurrence on other objects, such as easel paintings. An understanding of these materials is important since they can be responsible for significant changes in the surface appearance of artworks and the solubility of the matrices in which the oxalates are formed. The altered, oxalate-rich surface layers can present substantial challenges for the visual interpretation of the painted surfaces, the accurate scientific characterization of the original materials, and the development of appropriate and effective cleaning methods.