This manuscript is an English version of a paper contained in the Rapporto della Soprindendenza alle Gallerie, Bologna, 1967-1968, pp. 51-62. It concerns technical studies made on three Guercino paintings. One case is a painting of Sir William of Aquitaine taking the Habit, preliminary drawings of which indicated the presence of a pentimento of an arm underneath a section of sky. While x-radiography could not confirm this suggestion due to a thick layer of lead white, microscopic studies provided necessary confirmation. Another study was made of a painting of S. Bruno Certosino, and found that an unusually large amount of high quality ultramarine blue pigment had been used. Ultramarine blue was very costly, usually being supplied by the patron. A study of a painting of La Cattedra de S. Pietro focused on changes in Guercino's style over time. X-radiographs and microscopic studies are in accord with the documentary and visual evidence that the head of Christ in this painting was repainted by Guercino after a lapse of some 15 years.