The α, β, and γ phases of the GaAs(001)-(2×4) surface have been studied using rocking-curve analysis of reflection high-energy electron diffraction (RHEED) and reflectance difference spectroscopy (RDS). We have measured RHEED rocking curves and RD spectra from GaAs(001) under an As flux of 2.5×10-7 Torr, and have identified the temperature ranges where each of the α, β, and γ phases is stable. The 2×4 reconstruction was observed in the range of 480-600 °C. The β phase, which is stable in the range of 510-550 °C, has a β 2(2×4) structure, in good agreement with previous experimental and theoretical results. The γ and α phases are stable below 510 °C and above 550 °C, respectively. While the data from the γ phase are well explained by a mixture of β 2(2×4) and c(4×4) phases, we propose that the α phase has a β 2(2×4) structure, the atomic coordinates of which are slightly different from those of the β phase.