Heating under As-4 flux in vacuum is the conventional treatment of an epiready GaAs substrate before the MBE growth starts on it. As-4 protects the GaAs surface against thermal degradation when oxide desorbs during this heat treatment. The situation is different when an InP epiready substrate is being heated under the As-4 flux before starting the MBE growth of a ternary compound, e.g., InGaAs or InAlAs. The complete removal of oxide from InP requires annealing temperatures of 520 degrees C-530 degrees C. However, at these temperatures some monolayers of InAs still remain on the InP surface. This usually deteriorates the crystal quality of the ternaries grown on InP annealed at such low temperatures. Increasing the temperature over 530 degrees C removes InAs from InP. However, it also causes structural deterioration of the substrate surface; this has negative effects on the quality of the grown ternaries. This paper presents experimental results concerning technological measures which lead to considerable diminishing of the unfavorable effects for the ternaries caused by InAs film left on the InP substrate. We propose here to change the lattice mismatched InAs into less mismatched InAlAs through deposition of a small amount of Al atoms on InP during its annealing at 530 degrees C.