The GaAs(100) surfaces chemically treated in HCl-isopropanol solution and annealed in vacuum were studied by means of Auger electron spectroscopy, x-ray photoelectron spectroscopy, high-resolution electron energy-loss spectra, and Low-energy electron diffraction (LEED). Chemical treatment and sample transfer into ultrahigh vacuum were performed under nitrogen atmosphere. The HCl-isopropanol treatment removes gallium and arsenic oxides from the surface, with about 2 monolayers of excess arsenic being left on it. The residual carbon contaminations were around 0.2-0.4 ML and consisted of the hydrocarbon molecules. These hydrocarbon contaminations were removed from the surface together with the excess arsenic by vacuum annealing at 300-420 degrees C. With increased annealing temperature, a sequence of six reconstructions were identified by LEED: (1 X 1), (2 X 4)/c(2 X 8), (2 X 6), (3 X 6), (4 X 1) and c(8 X 2) in the temperature intervals of 250-400, 420-480, 480-500, 500-520, 520-560 and 560-600 degrees C, respectively. All surface reconstructions were irreversible. The structural properties of chemically prepared GaAs(100) surfaces were found to be similar to those obtained by molecular-beam.epitaxy-growth and by decapping of As-capped epitaxial layers.