With occasion of the “Murillo’s year” (fourth centenary of his birth), the Andalusian Institute for Cultural Heritage (Instituto Andaluz de Patrimonio Histórico) received the request from a private collector for the restoration and authentication of a bozzetto of the Murillo´s painting entitled Moses and the water from the rock of Horeb . During the intervention, several studies on the materials employed were carried out—on the bozzetto and on the final large dimension painting, to characterize the different components of the paintings (ground layer, pigments and binders), as well as to confirm/reject the authenticity of the bozzetto . After a detailed general inspection using multispectral techniques (visible, UV, IRR) some pentimenti were discovered. In addition, Macro X-ray fluorescence (MA-XRF) scanning technique was performed, complemented by point XRF and stratigraphic analyses. The results show that the ground layers were prepared in the usual fashion of the artist, using earths, calcium carbonate, iron oxide pigments and white lead. MA-XRF mapping allowed determining the spatial distribution and the combination of these pigments along the surface of both paintings. Results show that canvas, priming, binders, pigments and painting techniques of the bozzetto are compatible with those of the original painting, so we can conclude that most likely this is the original Murillo’s bozzetto .