The paper considers the effects of fluosilicates on stone decay. An exhaustive study of marble conservation was carried out on the arch of Alfonso of Aragon in Naples (Italy) using a variety of analytical techniques (optical and electron microscopy, x-ray diffraction, x-ray fluorescence, ion chromatography). Serious forms of alterations were observed, some of which are certainly related to the consolidation treatment with magnesium and zinc fluosilicates that the monument underwent in 1964.