Samples of altered layers found on limestone and marble surfaces were taken from San Petronio and other buildings in Bologna (Italy). Analyses (scanning electron microscopy, x-ray diffractometry, atomic absorption spectrometry) demonstrated the nature of the layers. Dolomite prevails in the external layer but is not found in unaltered rock. These observations are evidence that authigenic (penecontemporaneous) dolomite is formed on marble surfaces. The authors consider the role of microorganisms, ph and clay minerals in this process and suggest a model for the formation of the altered layer. Microphotographs, graphs, references.