The state of deterioration of Donatello's pulpit at Prato Cathedral was studied. Analytical data obtained show that its present condition is due both to natural causes of alteration which affect a sculpture standing for a long time in the open and to products used for restoration and conservation in fairly recent times. These products have caused reactions with the stone material and formed new mineralogical species which have been detected at moderate depths. They have been also a determining factor contributing to decohesion of the granules of the stone and hence to an increase in the specific surface, with consequent acceleration of the phenomena of alteration.