Nearly lattice-matched AlxGa1-xAsySb1-y alloys with Al composition x≫0.7 were grown by molecular beam epitaxy. The indices of refraction for a series of alloy compositions were measured. The Al composition was determined by x-ray diffraction (XRD) measurements assuming that the AlxGa1-xAs film that was grown prior to the growth of the AlxGa1-xAsySb1-y layer has the same Al composition. The arsenic to antimony ratio was determined by (004) symmetrical and (115) asymmetric reflection XRD rocking curve measurements. The refractive index and thickness were obtained by prism coupler measurements. A symmetric 75° Si prism coupler was employed to excite the waveguide modes with wavelengths of 1319 and 1540 nm. The shift in the measured indices of refraction from that of the calculated values revealed a deviation from the interpolating scheme based on the binary or ternary alloy data.