Algae are some of the least studied organisms in relation to building stone degradation, however, their importance in this field cannot be underrated. Algae, being autophototrophs are primary colonisers of natural stone alongside bacteria. Therefore, as well as damaging the stone themselves, they play an important role in creating the physical and chemical environments which the higher organisms later colonise. Laboratory studies have commenced to study the effect of photoautotrophs on the major sedimentary rock forming minerals, with a view to expanding this work to study the effect of these micro-organisms on heritage masonry. Tests have been carried out on albite, calcite, dolomite, orthoclase, siderite and quartz, using a mixture of axenic cultures of Chlorella vulgaris, Chlorococcum tetrasporum, Scenedesmus obliquus, Oocystis marsonii, Stichococcus bacillaris. The rock chips were immersed in either Autoclaved Tap Water (ATW) or Bolds Basal Media (BBM) and exposed to a mix of the microorganisms listed above, then tested weekly for their pH, fortnightly for the media’s chemical composition using ICP-AES, and visually utilising SEM and reflected light microscopy. Preliminary results have revealed biologically mediated etching of minerals, in particular carbonates and feldspars, a well defined pH profile over a period of 90 days, as well as a variety of elemental release patterns for Ca and Mg in association with different minerals. This in-vitro experiment showed that under these conditions there is evidence that the species of green algae used have a deteriogenic effect on the mineral substrate.