The etching of GaP, whether in strong HNO3 or in strong HCl is severely inhibited, whereas mixtures of HNO3/HCl are commonly employed as etchants for chemical polishing. The etching mechanism of n-GaP in aqua regia (3HCl/1HNO(3)) has been investigated. Aqua regia etching may occur by accomplishing both the following two processes: (1) oxidation and (2) dissolution. First, the nascent chlorine and nitrate ions obtained from the vigorous interaction between HCl and HNO3, may easily oxidize the surface atoms of GaP, then nucleophilic attacked by Cl- on the electron-poor trivalent gallium ions of the oxide lattice generally takes place over the passive surface. Moreover, the remaining phosphate or phosphorus oxide is very soluble. Ga and P atoms are thus carried away from the surface by the etchant system. In HNO3-etch, the nitrate ion formed is a weak nucleophile and the amount of its production is very small. In the HCl-etch, Cl- causes a direct attack on GaP substrate via an Ga(I) chloro intermediate whose thermodynamics is very unfavorable. Thus, the experimental observations are in good agreement with the mechanistic concepts presented.