GaAs1−xSbxnearly lattice matched to InP substrates has been grown by molecular beam epitaxy. For a given As and Sb flux, the GaSb mole fraction is shown to be sensitive to the Ga flux rate. Low‐temperature photoluminescence spectra exhibit a dominant emission line at 0.780–0.790 eV with a full width at half‐maximum as narrow as 7.6 meV, which is believed to be the narrowest obtained to date. Optical absorption measurements demonstrate that the band gap of this material is considerably smaller than predicted by the commonly accepted composition/band gap relation.