The surface structures of InP and InAs thermally cleaned in an arsenic flux were investigated using an ultrahigh vacuum scanning tunneling microscope multichamber system equipped with a molecular beam epitaxy facility. The InP surfaces thus treated at 510-degrees-C showed In-stabilized (4X2) reconstructions which depended on the time of thermal cleaning. The surface structure of this substrate after thermal cleaning for 3 min comprised one In dimer and three missing dimers. This structure was the same as that of InAs treated under the same conditions. These In-stabilized (4X2) structures formed on both InP and InAs substrates changed to an As-stabilized (2X4) structure when the substrate temperature was lowered to below 480-degrees-C under the same arsenic pressure.