Culzean Castle is located 45 miles south of Glasgow on the West Coast of Scotland. It is a fine example of the work of architect Robert Adam. The castle is currently owned by the National Trust for Scotland, and is one of Scotland's most popular tourist attractions. The stonework has suffered through prolonged exposure in the castle's cliff-top position overlooking the Firth of Clyde. As a result some of the stonework was replaced in the 1970s, and current with this research, more stonework needs to be replaced. The castle was constructed from local sandstones, specifically volcanigenic lithic arenites (quartz-rich sandstones containing feldspars, lithic fragments, clays and micas, cemented together with calcium carbonate). Culzean was built during three periods using local stone. The original Late-Medieval castle was constructed from what has proved to be 'durable sandstone' (OB) which shows little evidence of deterioration in its more sheltered location as the foundations of the current building. During the eighteenth century, Robert Adam designed additions to the castle using another local sandstone (AB). This stone has undergone considerable deterioration (e.g. spalling, contour scaling and surface pitting), and in some instances has failed, with surface material falling from the building. The majority of the AB stones, particularly on the Drum Tower balcony need to be cut back or replaced right away. Finally during the late 1800s a west wing was added to the castle, of redder sandstone from Maybole (VB), which currently exhibits some surface deterioration (mainly contour scaling), but not on the same scale as the AB stone. Research was undertaken to examine the differences between these stones, particularly to determine the characteristics of the AB stone which allow it to weather more rapidly than the others. Cores samples of 75 mm diameter, 150 mm depth were taken from the building using a dry drilling method to prevent chemical alteration. A number of analytical techniques including ion chromatography, x-ray fluorescence, x-ray diffraction, scanning electron microscopy and cathodoluminescence microscopy were used to examine the geochemistry and mineralogy of the stone. The role of externally derived soluble salts and the potential for physical change were then determined. Petrographic analysis highlighted a number of characteristic processes. The rock forming minerals, specifically the quartz, feldspars and micas showed similar mechanisms of deterioration in the three sandstones. These mechanisms included dissolution, particularly along lines of weakness and compositional variation, with ionic exchange at mineral edges. Clay minerals were found in abundance in the Culzean sandstones. Of greatest importance were the clays which absorb water, found in the AB stones (combinations of illite/smectite/chlorite species) which with access to ingressing solution and under confining pressures are particularly destructive for their size. There was also evidence for calcium carbonate cement dissolution and reprecipitation. The pore structure of the stones was of fundamental importance, as interconnecting pore channels increased permeability. The AB samples were found to have micro-porosity (< 5μ), created by cement and mineral dissolution, and interconnecting pore-space structures. Geochemical analysis identified the presence of soluble salts in the stone. Halite, gypsum and thenardite were present in abundance, particularly the AB samples. The occurrence of soluble salts at depth in the stone provided important information on the composition of the solutions driven into the stone at Culzean and the depths to which they could penetrate according to porosity and permeability. It also highlighted the presence of moisture at depth allowing salt crystallisation/hydration cycles and chemical reactions to take place. Analysis of the bulk chemistry of these three stones showed that overall the chemical make-up of each individual stone sampled was complicated and unique. There was no evidence for greater movement of specific elements through the stone profiles of the Adam's building stone when compared with the other two. Such was the natural chemical variation found in each of the stone profiles examined. The roles of temperature and humidity, controlling all aspects of moisture movement within the stone were also examined. These factors affected the ability of the stone to dry out and thus where the zone of maximum moisture content was situated and consequently where soluble salt crystallisation/hydration was heightened. There was surface evaporation of the exterior of the stone, behind which a wetting/drying zone allowed soluble salts to crystallise/dissolve. This salt crystallisation/solution zone moved in and out of the stone at depths of 10-40 mm, the zone of maximum moisture content. This resulted in the breakdown of the stone as its exterior surface spalled by the action of contour scaling. The results of this investigation have been used to produce a model for stone decay controlled by the ingress of salts in solution into the stone, and the ineffectual drying out of the stone.