During exposure to different fluids and temperature gradients, the shear strength and the stiffness of shales may change because of changes in their chemical and physical properties. Time-dependent borehole stability models should incorporate these changes in order to capture actual field conditions. In this study, analytical treatments are employed to predict changes in shear strength and stiffness of soft shales caused by concentration diffusion processes (hat may arise during drilling operations. Analysis is based on the double-layer theory and the modified effective stress concept. Artificial shale specimens with different smectite contents are prepared using a slurry-consolidation technique. Fluid exchange (higher tonic concentration) is imposed on the specimen before undrained triaxial and unconfined compression tests. Volumetric strains and pore pressures are recorded with time during pressure diffusion, concentration diffusion and shearing stages. Apparently, changes in the shear strength and stiffness depend on the initial condition of the system. Soft homogeneous sediments experience an increase in the strength and stiffness with the increase in pore fluid concentration. On the other hand, stiff cemented systems and clay mixtures may show the opposite trend due to the breakage of bonds and the generation of excess pore pressure.