This letter describes the performance of electro-absorption optical modulator structures, based on strain-balanced InGaAs/AlGaAs p-i-n multiple quantum wells (MQWs). It specifically considers the influence of lattice mismatch with the graded InGaAlAs buffer arising from unrelaxed material forming the substrate upon which the p-i-n structure is grown. We have shown that to achieve a lattice constant corresponding to the average for the MQW region, it is necessary to grade the buffer up to a higher indium concentration than the average indium concentration of the MQW. We have found that mismatched samples exhibit poorer modulation on the long wavelength side of the exciton peak as a result of a greater reduction in the exciton peak with applied field, and we attribute this to screening and nonuniform fields resulting from the ionization of defects at high fields. We have also shown that the similar effect induced by going to lower barrier potentials is relatively less important. © 1997 American Institute of Physics.