It is demonstrated that in optical waveguides formed by ion implantation in LiNbO3, the lower modes for the extraordinary index exist in a buried region of enhanced index, and so may be unable to couple with a surface prism. This is because a shallow region of slightly reduced index near to the surface necessitates the tunnelling of power to these ‘‘missing modes.’’ The number of such modes can be inferred from a comparison of data for two different wavelengths, and confirmation of this together with an estimation of their positions may be obtained by surface stripping. By taking them into account it has been possible to model the profile index by using a stepped base to the optical well.