We have studied linear electrooptic effects in long-range ordered (Al0.5Ga0.5)0.5In0.5P on GaAs (115)A and GaAs (001) by polarized electroreflectance spectroscopy. The electroreflectance spectra show anisotropic interference oscillations below the band gap. The amplitude of the oscillation signal linearly depends on the applied voltage modulated from the flat band. In strongly ordered (Al0.5Ga0.5)0.5In0.5P on the GaAs(001), an anisotropy of the oscillating signal caused by a difference of refractive indexes of ordinary and extraordinary direction was found. The amplitude of the [11¯0]-polarization spectrum is larger than that of the  polarization. Furthermore, the anisotropy becomes large with decreasing the detection energy. The dispersion of the anisotropic refractive index below the band gap depends on order parameter. © 1999 American Institute of Physics.