The quality of GaInAs-AlInAs epitaxial layers is found to be critically dependent on the degree of (100)-InP substrate misorientation. The alloy quality of both materials is improved when the substrate is misoriented 4°off the (100). The heterojunction interface quality as determined by the full width at half-maximum of quantum-well photoluminescence is also improved when a substrate misoriented by 4°is used. A degradation of both alloy and interface quality as compared to material on (100) InP is observed when the misorientation is 2°. These effects are also observed for strained quantum-well structures.