AbstractNatural stones are influenced by environmental conditions that might affect their performance and durability as building cladding elements. Among these environmental conditions, one that is not well explored is the study of the impact of SO2 deposition on the mechanical properties of ornamental stones. In this paper, the effect of a SO2-rich atmosphere (constant 10 ppm, with cyclic variations of temperature and relative humidity) on the physical-mechanical properties of some stones from Portugal (three granites and two limestones) is evaluated. Comparative studies on unaged and artificially SO2-aged samples were conducted. Samples were characterized by stereomicroscopy, scanning electron microscopy–X-ray energy dispersive spectroscopy (SEM-EDS), weight variation, and evaluation of elastic dynamic modulus through the fundamental resonance method and of flexural strength under concentrated load. Three granites—Amarelo de Vila Real, Cinzento de Alpalhão, and Cinzento de Pedras Salgadas—and two limestones—Moleanos and Semi-Rijo—from Portugal were chosen, as they are commonly used as cladding natural stones on façades. The obtained results showed that SO2 reacted with both silicate and carbonate stones; neoformed calcium sulfates were identified through SEM with different morphologies and in distinct levels. Its formation induced macro- and microstructural and mechanical modifications. Indeed, calcium sulfate crystallization critically affected mechanical performance through a reduction of elastic modulus and flexural strength in different degrees. Based on the flexural strength and dynamic modulus of elasticity variations, it is possible to recognize that only one granite, Cinzento de Alpalhão, presented lower failure risk in a cladding application. Thus, in urban SO2-rich environments, the effect of this pollutant on natural stone performance should be considered in the material selection stage, leading to improved cladding stone behavior and thus increasing stone service life.