A number of complementary experimental characterization tools and theoretical band structure methods were used to determine unambiguously the band-edge luminescence as a function of Al concentration, and to place an upper limit on the short-wavelength emission of InAlGaP alloys lattice matched to GaAs. In particular, the direct-to-indirect band-gap crossing has been determined by analyzing a series of metalorganic vapor-phase-epitaxy-grown In{sub 0.49}(Al{sub {ital y}}Ga{sub 1{minus}{ital y}}){sub 0.51}P alloys lattice matched to GaAs with double-crystal x-ray analysis, Rutherford backscattering spectroscopy, pressure- and temperature-dependent photoluminescence, and transmission electron microscopy. The experimental measurements are compared to first-principles plane-wave pseudopotential band structure calculations for the ternary end points, InGaP and InAlP. The maximum room temperature direct band gap is found to be 2.24 eV, corresponding to an Al composition of {ital y}=0.52{plus_minus}0.02, in good agreement with the theoretical prediction of 0.58{plus_minus}0.05. {copyright} {ital 1996 The American Physical Society.}