Heteroepitaxial films of (001) oriented GaxIn1−xP(x≊0.5) deposited on GaAs buffer layers have been grown by molecular beam epitaxy (MBE). Other than dislocations threading from the substrate, 1‐&mgr;m‐thick alloy films were free of extended defects in TEM examination for a range of 300‐K misfit strains between −7×10−4and 1.4×10−3. In contrast to VPE grown GaxIn1−xP, the absence of misfit related dislocations over such a range of misfit strains is believed to result primarily from the low MBE growth temperatures (670–790 K). TEM micrographs recorded using bright‐field, two‐beam conditions showed a grainy texture on the scale of 100 A˚ which may arise from group III sublattice inhomogeneity. Similar contrast is observed in high luminescence efficiency LPE GaxIn1−xAsyP1−yand VPE Ga0.5In0.5P. The photoluminescent (PL) efficiency from nominally undoped, 1‐&mgr;m‐thick films of MBE GaxIn1−xP(x≊0.5) was generally constant across typical wafer widths of 2.5 cm having a composition span of 0.02 inx. Deviations from this behavior may have resulted from variations in the group V to group III flux ratio from layer to layer. The maximum, external PL efficiency for near band edge emission for 1‐&mgr;m‐thick layers was 1.8×10−5. This low value is not due to competing radiative transitions between 1.9 and 0.5 eV since the low temperature PL spectra consist only of two dominant, near band edge peaks whose excitation dependence suggests they are donor band and donor acceptor in nature. They are probably associated with C and Si impurities, which have been detected by PL in the GaAs buffer layer. Both the 300 and 11‐K PL linewidths of nominally undoped films are broader than expected from thermal considerations alone and may reflect group III sublattice disorder. The dispersion of the refractive index of Ga0.51In0.49P has been determined in the range 700–1700 nm.