The degree of structural damage induced by chemical or ionic processes on the crystalline binary compound AB is estimated by means of X-ray photoelectron diffraction and using a model where the damaged layer A_2xB_2(1-x), out of stoichiometry, is assumed to be homogeneous and in an amorphous state. Applied to the case of acidic chemical etching of GaAs (001), the experimental angular distribution curves (ADC) matched with simulated curves obtained from crystalline ADCs give a disordered surface layer thickness and composition, d=0.6 nm and x=0.625 respectively.