Low‐temperature photoluminescence, photoluminescence excitation, and photoreflectance modulation measurements are used to probe the electronic structure and the dielectric function of (001) (GaP)2/(InP)2superlattices (grown on a GaInP2buffer) that exhibit lateral composition modulation. An 8×8 Luttinger‐model Hamiltonian, which includes the effects of coherency strain generated by the lateral composition modulation, is used to analyze the experimental results. These studies yield the first experimental evidence for a spontaneously generated effective mass superlattice.