The electronic properties of GaAsxP1−x/GaP(100) strained-layer superlattices with x<0.5 are studied using both a tight-binding model and an effective-mass model. By varying the alloy compositions and thicknesses of the layers, it is possible to independently vary the band gap and lattice constant of these structures. For the structures in which the bulk [100] minima are mapped into the strained-layer superlattice Γ point, a direct band gap occurs in the superlattice even though the bulk materials have indirect gaps. The results illustrate that strained-layer superlattices in general form a broad new class of semiconductor materials with tailorable electronic properties.