Inversion layers of 2&mgr;m and 30&mgr;m thickness were formed at the negative dipole-end(−c)face of aZ-cut LiTaO3crystal plate by proton exchange followed by heat treatment. The phase velocities of leaky surface acoustic waves (LSAWs) on both the positive(+c)and negative(−c)faces of the plate were measured by line-focus acoustic microscopy versus the angle of propagation, at a frequency of 225 MHz, and the results were compared with theoretical predictions. It is shown that for an inversion layer of 30&mgr;m thickness the phase velocities of LSAWs on the+cand−cfaces are the same, and almost equal to the velocities of the virgin plate. This suggests that the material constants, which are decreased due to the proton exchange, are restored to the original values by the heat treatment. ©1997 American Institute of Physics.