Smectites and hydrated Mg sulfate minerals have been identified in close association at various locations on the Martian surface. The hydration states of sulfates and smectites are dependent on temperature and relative humidity (RH), and therefore these minerals have the potential to affect cycling and bioavailability of H(2)O on Mars. We have conducted X-ray powder diffraction experiments to investigate cycling of H(2)O within mixtures of Ca-bearing smectites and hydrated Mg sulfate minerals under conditions of varying RH similar to those that exist at or just beneath the Martian surface. Our experiments show that under conditions of varying RH, cation-exchange reactions occur between these two potential components of the Martian regolith, producing gypsum [CaSO(4)center dot 2H(2)O] and bassanite [CaSO(4)center dot similar to 0.5H(2)O] in the absence of free-liquid H(2)O. Cation-exchange reactions were accompanied by significant loss of porosity, warping of the sample surface and, in some cases, volume expansion. The formation of Ca sulfate minerals in these experiments provides evidence for the development of thin films of H(2)O at mineral surfaces and suggests that similar processes may operate at the arid surface of Mars. Humidity-driven cation-exchange reactions between smectites and hydrated Mg sulfate minerals may therefore play a role in shaping the present-day Martian surface and could have provided a transient source of H(2)O and nutrients (e.g., major and trace elements and possibly organic micro/macronutrients) for putative microorganisms.