This article describes two effects that can cause serious bending in GaSb and InAs substrates used for quaternary compounds. Understanding of both effects is required in order to produce flat wafers. We have observed a very large bending with both positive and negative curvature in wafers with mismatched films, both at growth and room temperature. We therefore postulate that misfit dislocation formation is suppressed in these materials and that the bending is due to the lattice mismatch. However, bending may also occur in wafers where the mismatch in the center of the wafer is negligible. We propose that this effect is due to a mismatch due to nonuniform mole fraction variations across the wafer and throughout the film which in turn are induced by the way the flux distributions combine to create a particular mole fraction. The conclusion that must be drawn from these observations is that production of flat wafers require the intentional introduction of a slight mismatch during the growth process.