A study of the surface of InyAl1-yAs, grown lattice matched on InP(100) by molecular beam epitaxy, protected by an As cap during storage in air and subsequently annealed in ultrahigh vacuum. is presented. The surface structure and stoichiometry of the layers are investigated by low energy electron diffraction and x-ray photoemission spectroscopy. These investigations show that decapping may be achieved by annealing the sample at 390-degrees-C to reveal an atomically clean, As-stabilized surface, exhibiting a (3 X 1) symmetry. It is also shown that the relative population of In and Al on or near the surface is different from that of the underlying bulk InyAl1-yAs layer and that the overall surface symmetry and stoichiometry may be further adapted by annealing to higher temperatures.