Using x‐ray diffraction and transmission electron microscopy we have found that InAs1−xPxfilms deposited on InP(001) substrates with organometallic vapor phase epitaxy grow in an unusual island growth mode characterized by large strain‐dependent interdiffusion. Initially, strong intermixing occurs, producing pseudomorphic islands of intermediate composition. These grow only until some point in the relaxation process, possibly a critical value of the strain, after which islands of the intended composition begin to appear. Furthermore, both types of islands are found to penetrate deeply into the substrate. ©1995 American Institute of Physics.