The paper is a review of technical studies of the effects of solvents on oil paints in the context of removal of varnish from paintings. After an introduction describing the historical background to technical studies of cleaning, the various effects of solvents on oil paints are discussed: swelling and softening of the paint binder (which can contribute to the vulnerability of paints to pigment loss during cleaning); solvent diffusion and retention; and leaching (i.e. the extraction of soluble organic compounds from the paint). The paper closes with a discussion of methodological issues in cleaning studies, particularly the relationship between studies on model reference paint films and realistic, clinical studies of actual cleaning operations, considering also the related issue of ageing of oil paints.