Strained-layer multiple quantum well (MQW) InAsP/InP optical modulators have been fabricated from layers grown by metal-organic vapor phase epitaxy. The devices are a series ofp-i(MQW)-nphotodiodes in which the active core regions consist nominally of 25 periods of 10 nm InAsP quantum wells of 4.4&percent;, 10.0&percent;, 15.6&percent;, and 26.4&percent; As composition separated by 10 nm InP barriers. Structural parameters for the samples were obtained using high-resolution x-ray diffraction rocking curves and transmission electron microscopy. The series contains samples with both coherently strained and partially relaxed multi-layers where the relaxation is characterized by misfit dislocations. The band offsets for the heterostructures were determined by fitting the energy positions of the optical absorption peaks with those computed using the Marzin–Bastard model for strained-layer superlattices [as in M. Beaudoin &etal;, Phys. Rev. B53, 1990 (1996)]. The conduction band discontinuities thus obtained are linear in the As composition(7.56±0.08 meV per As &percent; in the InAsP layer) at low and room temperature for As concentrations up to 39&percent;, and up to 17&percent; average relaxation. Comparisons between the coherently strained and partially relaxed samples demonstrated a broadening of optical transition linewidths due to relaxation which appears to be of minor consequence for optical modulator devices as the essential optical and electrical properties remain intact. The electric field-dependent red-shift of then=1 electron-heavy hole transition was measured by a photocurrent method and found to be enhanced in structures with lower barrier heights. ©1997 American Institute of Physics.