The environmental factors affecting the chemical stability of objects in the collections of museums and historic sites include temperature, moisture, reactive chemicals in the air, and biological attack. The concentrations of deleterious airborne chemicals such as nitrogen oxides and sulfur oxides are usually a function of a museum’s proximity to cities, industrial sites and airports or to materials used in the building’s construction. These chemicals can be filtered and avoided. Pest and biological issues such as insect and rodent infestations and mold growth are also important issues but will not be discussed here. The mechanical instability of museum objects may be seen in cracking, delamination, flaking of paint, powdering, etc., and may be the result of chemical changes in the materials and/or environmental action. However, this paper discusses only the principal effects of moisture and temperature on mechanical and chemical stability of collections.