InxGa1−x−yAlyP, with y=0.0 to y=0.52, has been grown lattice matched to GaAs substrates using the technique of gas‐source molecular‐beam epitaxy. Growth conditions were established that resulted in conducting quaternary layers for both n‐type Si doped and p‐type Be doped samples. Si doped quaternary layers had a significantly reduced electron concentration at room temperature that varied with the AlP content in the InGaAlP layer. Intentionally doped InGaAlP layers exhibited room temperature photoluminescence for AlP mole fractions up to y=0.37; photoluminescence peak energies were 1.9 eV when y=0.0 (InGaP) to 2.3 eV when y=0.37 and decreased in intensity for increasing y.