Investigation of oxide chemistry and reactivity of III-V semiconductors has been undergone with an electrochemical approach coupled with surface analysis. Growth and stability of anodic oxides obtained on n-GaAs and n-InP was investigated with potensiostatic mode under illumination at pH 9. For the two compounds, the transient photocurrent. the recovery of the photocurrent and XPS analysis were performed in the same conditions. InP and GaAs present very different behaviours, which are linked to the stability of the oxidised interface. InP evidences a thin, stable and electrically blocking oxide film, while GaAs exhibits a thicker, unstable and porous oxide layer. This work point out that XPS analysis has to be coupled with in situ techniques as transient photocurrent, to understand accurately the oxide growth mechanism.