We demonstrate modulation for GaAs multiple quantum well (MQW) waveguide modulators on silicon substrates. The modulators, which were fabricated by molecular beam epitaxy, were operated at wavelengths of 890–910 nm, with greater than 20 dB modulation obtained at &lgr;=900 nm for a reverse bias of 2.5 V. Photocurrent measurements were performed on GaAs MQWs grown on Si and compared with results obtained for GaAs MQWs grown on GaAs. The structures were integrated with GaAs/AlGaAs waveguides and are suitable for integration with silicon‐based electronics.