Concrete exposed to sea-water may develop a surface skin causing a marked reduction in permeability. The skin typically consists of a layer of brucite (magnesium hydroxide) of around 30mum thickness, overlain by a thicker, but more slowly developing, layer of aragonite (calcium carbonate). The aragonite may convert to calcite in an aqueous environment free from magnesium ion. The degree of saturation of sea-water with aragonite or brucite is primarily a function of its pH value. Uncontaminated sea-water has a pH within the range of 7.8 to 8.3 and is supersaturated with aragonite at pH values greater than around 8.1. For good correlation of laboratory tests with natural exposure conditions appreciable increases in pH should therefore be prevented. Temperature variations may have a significant effect on layer precipitation, but the range of pressures encountered by offshore structures has negligible effect. Organic compounds present in sea-water my inhibit precipitation.