A Mach-Zehnder type electro-optic modulator based, for example, on GaAs/GaAlAs of InP/InGaAsP comprises coplanar stripline (CPS) electrodes, and optical waveguides, disposed on a conducting substrate or a conducting epitaxial layer, wherein the regions underlying the CPS electrodes are selectively rendered insulating by ion implantation to electrically isolate the CPS electrodes.