The paper presents room temperature photoreflectance measurements carried out on the Au/ultrathin Si/n-crystalline GaAs structure with the silicon interlayer thickness gradually changing from about 4-27 Angstrom prepared in situ under ultra-high-vacuum conditions. A continuous change of the phase shift of the corresponding photoreflectance signal of more than 180 degrees has been observed in the Eg-critical-point oscillation of GaAs depending on the Si interlayer thickness. The explanation of the effect is based on the composition changes of the Au/Si interface monitored by X-ray photoelectron spectroscopy. The photoreflectance results are compared to the photoreflectance signal phase changes observed on the semi-insulating GaAs surfaces exposed to the low-temperature hydrogen plasma. (C) 1999 Elsevier Science S.A. All rights reserved.