Conditions for congruent vaporization of GaAs, InAs and Ga y In 1− y As compounds under vacuum are calculated using thermodynamic data of basic binary and ternary systems. Erosion rates are predicted for the GaAs and InAs compounds that explain their tendancy to form liquid metallic droplets at their surface at high temperature. For the ternary Ga y In 1− y As compounds, partial congruent vaporization is defined that is useful for molecular beam epitaxy to avoid the droplet appearance at the compound surface. The maximal temperature for partial congruent vaporization is calculated as a function of the composition varying from pure InAs to pure GaAs. The curve has a maximum at 992 K for the Ga 0.99In 0.01As composition which explains the higher stability of these substrates under vacuum.