Two limestones from Crete, Greece and a dolomite from Mansfield, UK were subjected to combined microbial and physical weathering simulation cycles, in an attempt to assess the contribution of each agent of decay. Sound stone discs were exposed to different temperature and wet/dry cycling regimes involving treatment with distilled water or solutions of sodium chloride or sodium sulphate. Before the weathering cycles, half of the discs were inoculated with mixed microbial populations (MMP), originally recovered from decayed building stone of Portchester Castle, Hampshire, UK. The presence of MMP greatly accelerated the rates of deterioration of stone of all treatments, measured by weight change and alteration of hydraulic properties of stone. A combination of physical and biological processes significantly enhanced the extent of decay when compared with the physical or biological agents acting alone. Populations of heterotrophic, sulphur-utilising, halotolerant and moderately halophilic bacterial populations remained large throughout the experiment. Biofilms formed by populations of microorganisms were visualised by staining and assessed by colorimetric measurement of total carbohydrate in the stone substrate. The relative contribution of microbial and physical weathering to the process is discussed.