We report a novel technique to determine the index change in Li2O in‐diffused surface layer in LiNbO3by observing the cutoff angle of the extraordinary substrated mode as it refracts through the surface‐layer–substrate interface. Surface‐layer index changes from 10−4to 10−2were measured in various samples which received compensation treatments for times of 10 min to 6 h, primarily in LiNbO3powder. We also report the successful compensation of nearly single‐mode Ti : LiNbO3channel waveguides.