Lichens, formerly considered as weathering agents in a geological context, are shown to be capable of biodeteriorating stone substrata within a relatively short time-scale. Detailed Raman spectroscopic studies have demonstrated the highly destructive properties of calcium oxalate produced by lichen thalli. Dirina massiliensis forma sorediata is capable of chemically disturbing substrata to depths of almost 2 mm in under 12 years and converting the substrata into substantial quantities of calcium oxalate. Studies of disfigurement of English church walls assumed to have been rendered with a decorative or protective surface via restoration, have also been proved spectroscopically to be the result of this lichen’s biodeteriorative action. © 1997 John Wiley & Sons, Ltd.