Shells from several Mollusca collections have been examined. Efflorescence, white or gray crystal salts on the exterior or, to a lesser extent, the interior of the shells, is a widespread problem, and occurs in many shell species, both land and sea shells. The efflorescence has been identified by means of the following techniques: x-ray diffraction (XRD), infrared (IR) spectroscopy, thermogravimetric analysis (TGA), and NMR spectroscopy. The efflorescence products consist of 1 or a mixture of Ca(CH₃COO)₂* h1/2</sub>H₂O, Ca(CH₃COO)₂H₂O, and Ca(CH₃COO)(CHOO)H* l2*sO. When this mixed acetate-formate salt is dissolved from the shell and recrystalized, the product is almost invariably 3Ca(CH₃COO)₂ 4Ca(CHOO)₂5H₂O. Synthetic preparations of these novel calcium acetate-formate double salts aided in their identification. The nature and circumstances of the formation of efflorescence indicate AcOH and formic acid vapors, liberated by wooden storage cabinets, primarily oak, as the cause of deterioration. For certain specimens, coatings applied to the shells prior to storage have given long-term protection. Other forms of shell deteriorations are described briefly, and some methods of conservation and preservation are discussed.