Freshly quarried Doulting limestone of Jurassic age differs in terms of mineralogy and porosity compared to the decayed stone of Wells Cathedral, Somerset, UK. Stones from both localities contain allochems including ooids, peloids, extraclasts (reworked Carboniferous limestone) and bioclasts. Sparry calcite cement is present as twinned, syntaxial, overgrowths nucleated onto crinoid ossicles with high Fe content due to extensive burial cementation. The Fe content in specific phases determines the weathering stability of the rock on the building. The decayed stone shows the development of porosity which is parallel to the weathered surface, mobilization of Fe into a hard layer near the surface and incidences of high concentrations of SO42-, NO3- and Cl-. Despite low concentrations of SO2 present in the atmosphere at the time of the study, the extent of stone decay and composition of crustal deposits implies that SO2 pollution may have been greater in the past, with stone currently affected by gypsum salts formed during earlier pollution episodes.