Bulk GaAsP, 20Å-500Å GaAsP/GaInP single quantum wells and 70 Å period GaAsP/GaInP superlattices were deposited on GaAs substrates by Flow Modulation Epitaxy. In these structures, the disordered GaInP is lattice matched while the GaAsP is in tension with the As mole fraction varying from 0.6 to 1. The structures were studied using asymmetric x-ray diffraction, 1K photoluminescence, Raman scattering, transmission electron microscopy and atomic force microscopy. Raman and x-ray diffraction are used to assess the structural quality of the superlattices, especially with regard to the presence/absence of superlattice x-ray satellites and disorder activated longitudinal acoustic phonons. A model including the effects of composition, strain, and confinement on longitudinal optic phonons is described and used to estimate the composition, using Raman scattering, in the thin, pseudomorphic GaAsP layers in the superlattices. Photoluminescence is used to assess the composition of the interfacial layers in the single quantum wells and to determine transition energies in the superlattices. In addition, analysis of the heterostructure luminescence, including prediction of the energy band alignment as calculated with the model solid theory corrected for strain, is found to suggest the presence of a type II band alignment in the heterostructures for some values of GaAsP composition. Finally, Raman scattering and x-ray diffraction are used to compare arsenide to phosphide interfaces in GaAs and InP-based heterostructures.