A reductive deposition of 18-tungsto-diphosphate heteropolyanions (HPA) P2W18O6−62 is performed at n and p-GaAs surfaces. It is found by electrical measurements (I–V, impedance) and surface observations (SEM, TEM) that the treatment is more efficient and durable when it is performed at pH = 0. Photocapacitance measurements in that case reveal that the HPA surface modification induces surface states in the lower and the upper part of the band gap of GaAs. Such a distribution is able to explain both the observed improved H2 evolution at n and p-GaAs and the better stability of photoanodes.