Molecular‐beam epitaxy InAsySb1−ylayers were grown at temperatures ranging from 295 to 470 °C across the full composition range. Transmission electron microscopy and transmission electron diffraction (TED) examinations showed that for layers grown at and below 400 °C with nominal compositions 0.4<y<0.8, separation into two phases occurred resulting in a series of alternating plates approximately parallel to the layer surface. TED showed that the cubic lattices of the two phases were tetragonally distorted and their compositions were deduced to be typically InAs0.38Sb0.62and InAs0.72Sb0.28. The plates were larger and more regular along the [1¯10] direction than the [110] direction. As the growth temperature increased from 295 to 400 °C, for layers of nominal composition InAs0.5Sb0.5, the plate length increased from 0.1 to 2.0 &mgr;m and the plate thickness from 10 to 50 nm. Crystallographic defects were present in the layers and their occurrence was different in the phase‐separated and non‐phase‐separated layers. The plates formed spontaneously at the growing surface and were stable during subsequent annealing at 350 and 370 °C. It is suggested that they arise due to the presence of a miscibility gap at these growth temperatures. We have termed these spontaneously grown plate structures ‘‘natural’’ strained layer superlattices.