The Baptistery of Castiglione Olona (Varese, Italy), frescoed by Masolino da Panicale at the beginning of the 14th Century, is currently under restoration. On this occasion its state of preservation was investigated, in order to plan a correct intervention. During the sampling, an interesting phenomenon was pointed out, i.e. a wide rose coloration on the plasters of the interior northern and western walls. In order to clarify the nature of the alteration, a sampling was undertaken both for a chemical-mineralogical characterization of plasters (XRD, SEM-EDX, FTIR) and to detect the presence of biodeteriogens.