[en] We report on hard X-ray measurements with two epitaxial GaAs detectors of active areas 2.22 mm² and thicknesses 40 and 400 μm at the ESRF and HASYLAB synchrotron research facilities. The detectors were fabricated using high-purity material and in spite of an order of magnitude difference in depletion depths, they were found to have comparable performances with energy resolutions at -45 deg. C of ∼1 keV fwhm at 7 keV rising to ∼2 keV fwhm at 200 keV and noise floors in the range 1-1.5 keV. At energies <30 keV, the energy resolution was dominated by leakage current and electromagnetic pick-up, while at the highest energies measured, the resolutions approach the expected Fano limit (e.g., ∼1 keV near 200 keV). Both detectors are remarkably linear, with average rms non-linearities of 0.2% over the energy range 10-60 keV, which, taken in conjunction with Monte-Carlo results indicate that charge collection efficiencies must be in excess of 98%. This is consistent with material science metrology which show that the material used to produce them is of extremely high purity with impurity concentration <10¹³ cm^-3