Abstract Ionization processes in GaAs have been analyzed using primary ion beams (He +, Xe +) and primary neutral beams (He 0, Xe 0) with energies between 500 eV and 4 keV. The ionization probability of Ga + has been found to be independent of bombarding conditions and equal to 0.04. The As + and As − ionization yields depend very strongly on the bombarding conditions, and are the same for both ion and neutral particle bombardments. The absolute ionization probability of As + for Xe impact changes from 8 × 10 −7 to 2 × 10 −5 when the primary beam energy changes from 500 eV to 4 keV. Taking these experimental results into account, the most probable ionization process for As is suggested and discussed.