The microbial colonisation and contamination of rocks is basically controlled by environmental impacts, such as type of rock, climatic conditions and edaphic factors. Anthropogenic inorganic and organic air pollutants, specifically aliphatic and aromatic hydrocarbons, accelerate chemoorganotropic biodeterioration processes on exposed rocks of historical monuments. Besides the surface-controlled excretion of corrosive organic acids, stone-colonising microorganisms promote physico- chemical rock decay processes by the excretion of water absorbing extracellular polymetic substances (EPS) containing surface tension reducing compounds. Capillary water uptake and consequently the water holding capacity of rock is increased and possibly influenced by the above mentioned mechanism. The importance of these findings for the mechanisms of rock decay on historical monuments is discussed.