Films of In1−xGaxAs and GaSb1−yAsyover the entire composition ranges have been grown on (100) GaAs, InAs, and GaSb substrates by MBE.Insituobservations by high‐energy electron diffraction have revealed a variety of surface reconstructions and correlated the growth process with the lattice mismatch. The compositions are governed by the relative rates of In and Ga in In1−xGaxAs, but primarily by that of Sb in GaSb1−yAsybecause of its dominant incorporation over As. In these alloys, Sn is found to be a donor throughout In1−xGaxAs but an amphoteric impurity in GaSb1−yAsy.