The mean energy necessary to generate an electron-hole pair in gallium arsenide by x and γ photons has been measured in the 230–320 K temperature range. The experimental apparatus consists of a Schottky junction on a high-quality epitaxial GaAs, a silicon detector that generates a reference charge signal and highly stable low-noise electronics. The resolution of the system in measuring the generation energy is better than 2 meV. An apparent value of ε=4.216 eV has been measured at 300 K, lower than previously published values of 4.35 and 4.27 eV for α particles and 4.57 eV for electrons. Charge trapping in the GaAs detector, although very weak (less than 1%), has been observed and a model, based on the Hecht theory, has been developed to derive the true value of ε=4.184±0.025 eV at 300 K. The dependence of ε on the temperature, in the 230–320 K range, has been found linear, ε=4.55-0.00122×T [eV]. The dependence of ε on the band gap has been determined ε=2.76×Eg+0.257 [eV], in good agreement with the Shockley-Klein theory. At 300 K, the found value of ε in GaAs, together with those reported for germanium and silicon, yields a linear dependence of ε on the band-gap energy ε=1.83×Eg+1.6 [eV], with a correlation coefficient of 0.9997. © 2002 American Institute of Physics.