The effect of acid rain on structures has been illustrated by the example of the deterioration of the exterior of the Field Museum. The survey revealed that the marble cornices, etc., protected from the direct impact of rain have developed black crust. The unprotected areas however are clean due to perpetual dissociation of calcite grains. Fractures in marble blocks were found along dark bands which terminated at joints with polymeric caulking. Atomic adsorption spectrophotmetry, scanning electron microscopy, and x-ray diffraction of samples from the museum revealed that the black crust consisted of gypsum which had formed due to sulphur dioxide attack on calcite. The gypsum was absent at the naturally cleaned surface. By comparing these findings with the deterioration of marble at other antique structures it has been concluded that the protected regions shall, eventually, suffer greater deterioration. The conservation technology is yet in its incipient state. It is perhaps more economical to clean the air and save less deteriorated structures from meeting the fate that has visited the ancient marvels of architecture.