A large anomalous attenuation and depolarization scattering have been observed in

-cut,

-direction propagating LiNbO
3proton exchanged waveguides at 0.632 μm wavelength. We observe this phenomenon only when the mode effective index is smaller than the ordinary index

. The effect is caused by phase-match scattering into the substrate. Exchange solutions with various mole percentages of lithium benzoate have been used and the same phenomenon has been consistently observed.