The use of materials and painting technique in nine, supposedly darkened oil paintings (dating between 1834 and 1920) were studied with a variety of analytical techniques. By analyzing a large number of samples, some relationships between materials and darkening were found. The darkened areas of the paintings contain non- (or only slightly) heat-bodied linseed oils, with the exception of one painting by Rousseau, in which heat-bodied oil-containing paints were applied. Small amounts of beeswax were found as additives in some paints; lead dryers were detected in all paintings. A wide variety of pigments was used. The paints are relatively medium-rich and transparent in the dark(ened) areas, indicating that yellowing of the medium predominately contributes to the darkening. The severe surface deformations in some of the studied paintings are related to the painting technique, involving the application of many layers of medium-rich and slow-drying paint. -- SCMRE