The linear electro‐optic coefficientr41was determined in quaternary double heterostructure waveguides containing ap‐njunction. Over the limited range of measurements, it is independent of wavelength and composition with a valuer41=−1.4×10−10cm/V. A strong quadratic electro‐optic effect has been identified for the first time, which strongly depends on the band gap and photon energy as does the associated electroabsorption effect.