Art's shine may be restored.
The findings, from a study of samples from two van Gogh paintings, may lead to new ways of preserving or restoring old masters.
He loved the chrome yellow pigment that gave his "sunflower" paintings such intensity.
But since the early 19th century it has been known that chrome yellow is affected by sunlight, darkening to a brown.
New X-ray techniques have now identified the cause of the problem, a "reduction" reaction that alters the chromium in the paint.