This study shows, on the basis of a mode theory, that as a Gaussian ultrasonic beam strikes the edge of a thick plate immersed in a liquid, a Stoneley wave is generated along the liquid/solid interface. As expected, the largest amplitude for the excited Stoneley wave is obtained for normal incidence at the edge but a secondary maximum appears at Rayleigh angle incidence. In this way, it is proved theoretically that the conversion phenomenon of a Stoneley wave into a generalized Rayleigh wave at the extremity of a fluid loaded thick plate is reversible, as was observed experimentally by Tinel [Ph.D. thesis, Université Le Havre, France (1991)].