Ultraviolet-visible reflectance spectroscopy is a rapid and accurate technique for the investigation of painted surfaces. The use of a high efficiency beveled-tip fiber optic probe and a pulsed xenon source gives improved detection efficiency and minimizes the interference from ambient light, eliminating the need to shield the analyzed area and allowing non-contact, non invasive analysis. Thin washes of organic colorants, often impossible to sample, are ideally suited to this kind of analysis. This study describes the in situ identification of indigo, Prussian blue and a red insect dye (cochineal carmine or lac dye) in two paintings by Katsushika Hokusai