Numerous prisms of four different igneous and four different sedimentary rock types were measured over 35 wetting/drying cycles in KCl and NaCl solutions to establish dimensional changes which would adversely affect the material's serviceability. The major fact affecting dimensional changes was found to be the action of the various cations on swelling clay minerals, the degree of hydration of the minerals being directly influenced by the type of cation present on the exchange site. Recognizing the roles of porosity, degree of induration, and placement of constituent minerals it reports, generally for the samples tested, that KCl decreases or retards swelling when compared with NaCl. The use of potassium cations to improve the stability and behavior of rocks is suggested.