In this work, the process of phosphorus desorption from epi-ready InP(001) substrates during high-temperature annealing in an arsenic flux was experimentally investigated. An InPAs solid solution and InAs islands were formed on the surface upon annealing. The composition of the solid solution, the surface area fraction occupied by InAs islands, and its height depend on the annealing temperature. The phosphorus desorption rate was determined from the dependence of the arsenic atoms number on the substrate surface on the annealing temperature and the holding time in the arsenic flux. The phosphorus desorption rate increased from 6.03×1010 s−1⋅ cm−2 at an annealing temperature of 500 °C to 4.38×1011 s−1 ⋅ cm−2 at 540 °C. The activation energy of the phosphorus desorption process was 2.7 ± 0.2 eV.