The ternary‐phase diagram of GaAsSb has been calculated using Darken's quadratic formalism for a ternary liquid and assuming a regular solid solution. Liquid epitaxial layers of GaAsxSb1−xhave been grown in the range 0.75>x>1 on {100} and {111} GaAs substrates. Results are in excellent agreement with the calculated phase diagram. Variation of bandgap with composition of the layer has been determined by transmission, photoemission, and x‐ray fluorescence experiments. The data were fitted to a curve of the formEG=A+Bx+Cx2, whereA=0.725 eV,B=−0.32 eV, andC=1.02 eV. Graded bandgap layers have been obtained, with gradients of 700 eV/cm near the substrate interface and 25 eV/cm for thick layers. For use as high‐efficiency photoemitters, the samples were dopedptype by the addition of elemental Zn to the melt. Cesium and oxygen surface layers were used to lower the work function. Quantum yields of 0.1%–0.2% at 1.06 &mgr; were obtained. Field assisted photoemission in a graded bandgap sample has been calculated and demonstrated experimentally.