The As/P exchange behavior at interfaces of As compound/P compound heterojunction superlattices has been assessed by observing changes in average lattice spacing when very thin but different thickness As compound layers are inserted into InP, or P compound layers inserted into GaAs. Compositional changes are inferred from strain behavior measured by x-ray diffraction. Implications of the data are : (1) As rapidly displaces P and P slowly displaces As; (2) As atoms displace P atoms and reconstruct as a discrete single layer above 600 degrees C and as a double layer below 600 degrees C when InP surfaces are exposed to AsH3; and (3) interfaces and layers of different P compounds on GaAs show the same temperature-dependent inclusion of As.