We study using first-principle calculations the electronic and optical properties of In0.5Ga0.5As/InP and In0.5Al0.5As/InP superlattices, where the InGaAs and InAlAs alloys are described through an appropriate ordered ternary structure. The calculated electronic properties show that the substitution of Ga with Al originate an opening of the band gap from the infrared to the near visible and a transformation of the band alignment from type I to type II. Through the analysis of the optical properties we discuss successfully the giant polarization anisotropy observed in these systems. (C) 2001 Elsevier Science B.V. All rights reserved.