Abstract: A slow-wave electrode structure suitable for use in integrated optical modulators having a pair of spaced substantially parallel integrated optical waveguides in a compound semi-conductor substrate (such as gallium arsenide (GaAs) or indium phosphide (InP)) is formed by a pair of substantially parallel conductor strips on the surface of the substrate each in a position for interaction with its respective adjacent of said waveguides. The conductor strips are capacitively interconnected by narrow fins each integral with one of the strips and extending between the strips whereby the conducting strips and fins may be applied as a single layer to the substrate. The narrow fins significantly increase the capacitance per unit length between the strips without an equivalent decrease in inductance per unit length along the strips such that the phase velocity of the microwave signals is reduced.