A systematic study of the nature of arsenic incorporation in GaN grown by molecular-beam epitaxy is presented. The samples were grown with concentrations of arsenic ranging from 3.4 × 1017 to 4.2 × 1018 cm-3. Secondary ion mass spectroscopy data show that increasing the As concentration has the effect of increasing the amount of As in the nitrogen site as compared to As in the gallium site. This trend is used to explain the reduction in carrier mobility with increasing As concentration. © 2001 American Institute of Physics.