For a broad range of intermediate values of the hydrogen chemical potential, ∆µ H ∼ -0.2 • • • -0.8 eV, the 2D-2H surface is found to be stable. It corresponds to a 2×2 reconstructed surface formed by P dimers, see Fig. . Hydrogen is adsorbed in an alternating sequence on these buckled P dimers. Similar structures were observed for the (001) surface of InP, GaP, and In x Ga 1-x P prepared by gas phase epitaxy . The P dimers of the 2×2-2D-2H surface break up at extremely H-rich conditions, ∆µ H ∼ 0 eV. In this case, the surface P atoms are alternatingly decorated by one and two hydrogens, forming the 2×2-4P-6H structure. The hydrogen-covered surface P atoms are replaced by H-decorated In-P and Al-P hetero-dimers, if the surface is prepared in hydrogen and cation-rich conditions.