The miscibility gay of molecular beam epitaxial grown InAs1-xSbx layers on GaAs (0 0 1) substrates is investigated using X-ray diffraction techniques and scanning electron microscopy. Four different buffer layers, GaAs, InAs, GaSb, and homogeneous InAs0.5Sb0.5 grown at 470 degreesC, are used to investigate the strain effect on phase separation. The nominal composition of InAs1-xSbx is adjusted to x = 0.5. The layers grown at relatively low temperatures show two different alloy compositions and consist of alternating plates approximately parallel to the substrate surface, both indicating the occurrence of phase separation. The critical temperatures are 415 degreesC for the alloys grown on the GaAs and on the homogeneous InAs0.5Sb0.5 buffer layers and 435 degreesC for the alloys grown on the InAs and on the GaSb buffer layers. They are much higher than the temperature predicted by the delta lattice parameter model (303 degreesC). We believe that 415 degreesC is the critical temperature which is purely determined by the excess entropy and enthalpy with no respect to the substrates. The use of substrates such as InAs and GaSb, whose lattice constants are close to those of the phase separated alloys, promotes the phase separation due to the strain effect.