Total-energy and electronic structure calculations based on density-functional theory are performed in order to determine the atomic structure and electronic properties of Al \(_{0.5}\) In \(_{0.5}\) P(001) surfaces. It is found that most of the stable surfaces obey the electron counting rule and are characterized by surface atom dimerization. The dimer related surface states are predicted to occur in the vicinity of the bulk band edges. For a very narrow range of preparation conditions, ab initio thermodynamics predicts metal atomic wires formed by surface cations. In case of typical metalorganic vapor-phase epitaxy growth conditions, a surface covered with a monolayer of buckled phosphorus dimers, where half of the phosphorus atoms are hydrogen-saturated, is found to be stable. This structure also reproduces nicely the reflectance anisotropy measured for epitaxially grown AlInP(001) surfaces.