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 &mgr;m × 19 &mgr;m × 2 mm. Two electrode configurations giving two field orientations are used. Waveguide modulators withV&pgr;(voltage for &pgr; 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.