The grounds of twenty-six paintings by Nicolas Poussin have been studied. Their colour and chemical composition can be correlated to the artist's career. During his first stay in Rome, Poussin composed his work on brown grounds consisting of chalk and brown earth pigments. During his brief stay in Paris he used a red ground with a composition similar to that used by all the Parisian painters of the time. After his return to Rome and for the rest of his career, Poussin prepared his canvases with red ochre, except for landscapes, where he continued to use brown grounds.