The interfacial recombination of InxGa1−xP/GaAs heterojunctions is measured as a function of lattice-parameter mismatch for values of strain between 3×10−4 and 2.5×10−2. It is found that when InxGa1−xP is epitaxially deposited on GaAs such that the lattice parameter is larger than the underlying substrate, the interfacial recombination velocity s varies linearly with mismatch according to the expression s?2.5×107ε cm sec−1. This behavior can be quantitatively understood in terms of an increasing density of dislocations acting as recombination centers at the interface. When the InxGa1−xP epitaxial layers have smaller lattice parameters than the underlying GaAs, such that ε≳3×10−3, there is a very high recombination velocity and degradation of the electronic properties of GaAs. This is due to the formation of cracks in the InxGa1−xP which form dislocations in the underlying GaAs. For small lattice mismatch, ε<1×10−3, there is an appreciable recombination velocity which cannot be explained in terms of dislocations.