Guided‐wave electro‐optic Mach–Zehnder interferometric modulators have been fabricated on LiNbO3. The channel waveguides were formed by titanium indiffusion in some devices and proton exchange in benzoic acid in others. The modulators were tested at 0.85‐&mgr;m and/or 1.3‐&mgr;m wavelengths for insertion loss, electro‐optic activity, and susceptibility to optical damage. A substantial reduction in the electro‐optic activity accompanied by a reduced susceptibility to optical damage is measured for the proton‐exchanged modulators.