Amorphous layers of gallium arsenide (a-GaAs) formed by heavy implantation of silicon ions and crystalline gallium arsenide (c-GaAs) were irradiated with monochromatized X-rays using brilliant synchrotron radiation. Infrared absorption measurements at low temperature for a-GaAs specimens showed that X-rays having an energy larger than the K-binding energy of As atoms created a much larger fraction of Si-Ga and Si-As bondings than in the as-implanted state. On the other hand, from photoluminescence measurements, it was confirmed that X-rays having a smaller energy than either of the K binding energies, enhanced the relaxation of the a-GaAs network, and created some defects in c-GaAs. The mechanism for these structural changes is discussed from the viewpoint of relaxation processes after inner-shell electron excitation by X-rays.