We studied the reactivity of GaAs in;contact with acidic solutions of, respectively, four heteropolyanions (HPAs): SiW12O404-, SiMo12O404-, PW12O403-, and P2W18O626-; The results obtained by both x-ray photoelectron spectroscopy and etching rate and absorbance measurements showed that GaAs dipped into such solutions undergoes an oxidation/dissolution process while the HPAs are reduced. Indeed, after each immersion of GaAs into an heteropolyanionic solution, a deposit, which consists of As-0 atoms with either partially reduced HPA or WO3 is observed. This study has given us the opportunity to determine the reactions which occur at the GaAs/''HPA solution'' interface and to point out the importance of the semiconductor energy-band position with respect to the first redox potential of the HPA. Knowledge of the semiconductor/HPA energetic dia,sram allows one to predict the behavior of HPA species toward semiconductors.