Presents the results from a color infrared photographic study of the occurrence of ultramarine and azurite in a group of 13th- and 14th-century Sienese panel paintings. Color infrared film is a so-called false color film that records the two visually similar blue pigments, ultramarine and azurite, as different, distinct, colors. This photographic technique was used to examine a number of early Sienese panels in European collections, including works by Guido da Siena, Duccio, Simone Martini, the Lorenzetti, and several later 14th-century masters. Two aspects of technique are explored in the article: the relative uses of ultramarine and azurite in three generations of Sienese panel painters spanning the years ca. 1260-1345 and the creative use of the full range of grades of ultramarine by the later 14th-century masters. These findings are discussed in the context of the aesthetic and economic values placed on the two blue pigments during the late medieval and early Renaissance period. Ultramarine was more highly prized than azurite on account of its tonal richness, but it was also very expensive and, therefore, generally used more sparingly.