The extent of shear failure in wet sand was monitored on a horizontally vibrating plane. The sand was contained in an open box, and normal stress was imposed by a top plate and a spring-loaded system. Accelerometers were mounted on the base and the top plate. The shape of the accelerometer output curves enabled shear failure to be detected. Failure was either internal or at the sand/base interface. The coefficient of friction, mu, was calculated from the peak acceleration of the top plate, and was found to be dependent upon the time and stress history. Mu increased as the system sheared, until such a point that failure ceased and the sand ’caked’ onto the vibrating base. The strength of the sand was just sufficient to withstand the applied vibrational stresses. When these stresses were increased, the ’caked’ sand failed in shear. Subsequently, mu increased until failure ceased at a higher value, corresponding to the new stress regime. Plots of shear stress vs. normal stress have been produced, but these could not be interpreted by simple relationships.