Arsenic 2p core-level shifts at GaAs/Al2O3 interfaces are determined with respect to bulk GaAs through density functional calculations. Atomistic interface models are constructed in which As atoms are found in various chemical environments. Both Ga-terminated and As-terminated GaAs substrates are considered, but only the former are found to be consistent with experimental data. The shifts of several oxidation states of As are calculated and a good agreement with experiment is found for the As+3 and As+5 states. Interfacial As-As dimer atoms and As-Ga antisites are investigated as candidates for the experimental photoemission line assigned to As-As bonds. The calculated shifts favor an assignment to As-Ga antisites.