Portland Brownstone (PB) is a widely used building material, particularly in New York City, that is susceptible to severe deterioration from weathering. This stone contains clays that may cause damage by dilating during wetting/drying cycles. We have measured the swelling strain of PB in aqueous solutions to determine the effect of ionic strength (of KCl and CaCl2) and water activity (in isopropanol solutions). We have also measured the stress exerted during swelling when the expansion is constrained. These data permit calculation of the stresses produced during wetting/drying cycles, which are found to be comparable to the tensile strength of the stone.