Mollusk shells are composed chiefly of calcium carbonate (CaCO 3</sub), either as calcite, a six-sided crystal, or as aragonite, a flat thin crystal. Deterioration is characterized by white to gray crystals or powder which obscure detail and pit the surface, so-called Byne's disease. This encrustation may be calcium acetate, formate, chloride, or complex salts of acetates and formates. Oak storage cabinets, plywood, chipboard, newspaper, other wood products and formaldehyde resins are sources of the organic acids. Marine shells are more commonly affected than land shells owing to formation of deliquescent chloride salts, e.g. CaCl 2</sub>, which absorb acetic and formic acids, and supply H 2</sub>O necessary to help convert carbonates to corresponding salts. Deterioration of natural history specimens is chiefly due to improper storage conditions, and probably aggravated by chloride contamination. Deterioration of archaeological shells may be complicated by burial in acidic clay soils which convert CaCO 3</sub> to watersoluble Ca(HCO 3</sub>) 2</sub>, leading to a porous layer structure with increased surface area available for reaction. Shells built up of aragonite crystals are more dense and survive burial better than less compact calcite. Proper storage in an inert low RH environment and possible conservation treatments are described.