Properly designed GaAs&sngbnd;AlxGa1−xAs double heterostructures produce strong optical waveguides. The propagation constants of the waveguide modes can be readily modulated by the linear electro‐optic effect. Measurements at a wavelength &lgr; = 1.153 &mgr;m have yielded a phase modulation of 180° with ‐10 V applied bias to a device only 1 mm long. The power necessary to phase modulate light at &lgr; ≈ 1 &mgr;m by 1 rad is of the order of 0.1 mW per 1‐MHz band‐width. The power dissipation is very strongly dependent on wavelength. At present, the high‐frequency modulation is limited by the series resistance and capacitance of the device. The highest cutoff frequency determined thus far, ≈ 4 GHz, is considerably lower than that calculated based on the geometry and material properties.