Experimental results are presented to show how the compressive strengths and acoustic velocities of different types of shale change when they are exposed to water-based fluids. The acoustic velocity (or slowness) and compressive strength of a soft, high water activity, Pierre I shale increase after exposure to different ionic solutions, while for the lower water activity Arco shale, the converse is true. By combining these tes ts with a new gravimetric test that quantitatively determines the flux of water and ions into and out of the shale, it was shown that these different effects correlate well with the movement of ions and water into the shale. In every case, water adsorption weakened, while ion adsorption strengthened the shale. The influence of salt type and salt concentration on the strength and acoustic velocity (slowness) of two shales was also investigated. It was observed that the ionic content of the water-based fluid had a significant effect on the changes in shale properties. These changes in acoustic velocity and compressive strength are highly correlated. This suggests it may be feasible to use acoustic logging data to determine changes in the mechanical properties of shale. Finally, the impact of the reported changes in the mechanical properties of the shale on wellbore stability was demonstrated through the use of a 3-D wellbore stability analysis program.