Measured effective mode indices on a day to day basis reveal an oscillatory behavior which indicates that the index profile of proton‐exchanged LiNbO3waveguides evolves as a function of time. It is suggested that the profile instability might be brought about by a continuous migration of protons within the thickness of the guiding layer. Some index profile modificaiton may also be introduced during fabrication due to structural changes in the crystal as a function of exchange time.