The changes in appearance of Turner's oil paintings have been studied, as part of a project to investigate the materials and techniques which he used. Not all paintings show evidence of faded organic pigments, or loss of color in newly manufactured, unstable ones, though this is responsible for the greatest alterations when it does occur. Less widely recognized factors, such as the darkening of the priming, sinking-in of paint, darkening of the paint medium, and other optical changes, and better-known ones such as the effects of yellow varnish and the consequences of lining or past overcleaning make important contributions too. The effects and relative magnitudes of these factors are discussed, and little-altered paintings are identified.