Cohesion debonding and friction mobilization along discontinuities are studied in the laboratory by monitoring seismic wave propagation through a discontinuity and measuring slip displacements using Digital Image Correlation (DIC). Slip was imposed on two individual blocks of gypsum through a contact surface that had both cohesion and friction. Investigation of slip was carried out by acquiring transmitted and reflected compressional, P-, and shear, S-, waves prior to and during failure of the specimens. DIC was used to monitor the discontinuity by measuring the surface displacement field. Direct shear experiments performed on gypsum specimens with cohesion revealed that debonding (loss of cohesion) occurred before the peak shear strength was reached, and that friction mobilization and cohesion debonding do not occur simultaneously along the discontinuity. In fact, the amount of slip that is induced due to debonding is an order of magnitude smaller than the slip needed for mobilizing the full frictional strength of the discontinuity. The loss of cohesion has distinct seismic signatures and resulted in a significant decrease in transmitted amplitude, an increase in the reflected amplitude, a significant reduction in wave velocity, and a sudden reduction of the dominant frequency of the waveforms for both compressional and shear waves.