(AlxGa1−x)0.51In0.49P layers, lattice matched to (001)‐oriented GaAs substrates, have been grown throughout the entire aluminum composition range fromx=0 to 1.0 by atmospheric pressure organometallic vapor‐phase epitaxy (OMVPE), using trimethylaluminum (TMAl), trimethylgallium (TMGa), trimethylindium (TMln), and phosphine (PH3) as source materials in a horizontal reactor. The growth temperature was held constant at 680 °C. Excellent surface morphologies were obtained over the entire composition range. Unlike previously reported results, neither high growth temperatures nor low pressures were needed in order to obtain good‐quality, high‐x(AlxGa1−x)0.51In0.49P alloys using trimethylalkyls. Photoluminescence (PL) was observed, even at 300 K, for all samples with Al solid compositions ofx≤0.52. The results show that the energy band gap measured by PL at room temperature for this material varies asEg=1.9 +0.6x, in accord with previous studies. It was found that the PL emission intensity was nearly constant at 10 K with increasingxin the range from 0 to 0.52. This contrasts with earlier published results which showed a decreasing PL intensity for the higher values ofx. The 300‐K PL intensity was almost a constant forx≤0.3 and gradually decreased with increasing Al content forx>0.30. The dependence is nearly that predicted from a simple calculation based on the relative occupancies of the &Ggr; andXconduction bands using a constant minority‐carrier lifetime. The PL full width at half maximum (FWHM) forx=0 was 7.2 meV at 10 K and 35 meV at 300 K. These are the narrowest reported results to date. Forx=0.48, the FWHM was 31 meV at 10 K and 78 meV at 300 K. It was observed that at both 10 and 300 K, the FWHM increased slowly with increasing aluminum concentration.