The electrochemical response of GaAs, n- and p- type, in the presence of a Keggin-type heteropolyanion-SiMo12O404- has been studied in order to determine the reactions which take place at the GaAs/SiMo12O404- solution interface. The behavior of the GaAs electrodes, in the dark and under illumination, allowed us to determine the nature of the semiconductor carriers involved in the charge transfer. All the charge transfers occur via the GaAs valence band (VB). At open circuit potential, the SiMo12O404- ions are reduced at the GaAs surface which therefore undergoes an anodic dissolution process. Under polarization, when the reduction current is maximum, no GaAs oxidation reaction takes place. The SiMo12O404- reduction current densities are the same at a platinum electrode and at a GaAs electrode. Those observations imply that at a GaAs/SiMo12O404- solution interface, under electroless conditions, the GaAs anodic dissolution occurs via a purely electrochemical process, by consumption of the holes injected during SiMo12O404- reduction.