Electro-optic intensity modulation is demonstrated in three-dimensional (channel) waveguides fabricated by diffusion techniques in ZnSe and CdS. Typical diffused guide dimensions are 1.6 μm × 19 μm × 2 mm. Two electrode configurations giving two field orientations are used. Waveguide modulators with V (voltage for π radians phase shift) of 72 V with rise times less than 5 nsec are described. Waveguide modulation in epitaxial layers of ZnS and ZnSe on GaAs is also described.