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Analysis of two clay-rich sandstone blocks from a temperate maritime, urban environment indicates that, although structural changes such as scaling and flaking affect only the outer 10-20 mm of stone, the zone of alteration extends well beyond this to a depth of up to 60 mm. This zone of alteration coincides with chemical changes, in particular the accumulation of calcium, sulphate, sodium, magnesium and chloride identified by ion chromatography and atomic absorption spectrometry analyses. Below 60 mm, calcium and sulphate concentrations decrease rapidly and remain relatively uniform throughout the rest of the substrate, while sodium and chloride appear to have a greater mobility within the stone penetrating to, and being retained at, much greater depths. X-ray diffraction analysis of samples from the zones of chemical and structural alteration identified halite and gypsum salts. However, although these changes affected both samples and both had similar exposure histories, only one of the blocks examined exhibited obvious signs of decay through granular disintegration and multiple flaking associated primarily with the exploitative action of accumulations of halite and gypsum salts. Salt weathering is a threshold phenomenon where decay is manifest only when a stress/strength threshold is crossed. Consequently, in a structurally and mineralogically variable sandstone, differences in porosity and/or clay mineral concentrations may result in different blocks being more susceptible to decay than other blocks despite similar accumulation of salts and exposure to the same environmental conditions. Copyright (C) 2000 John Wiley & Sons, Ltd.