We present evidence for anomalously large, strain-dependent interdiffusion inInAs1−xPxlayers grown on InP(001) substrates by organometallic vapor phase epitaxy at 620 °C. Specifically, there are strong indications for the existence of a “critical strain:” if the strain is∼1.9&percent;or more, much P–As mixing occurs, but for smaller strain the mixing is greatly decreased. The interdiffusion is also highly sensitive to temperature. A set of samples grown at 580 °C exhibits a factor of∼2decrease in P–As mixing compared to a set grown at 620 °C. ©1997 American Institute of Physics.