Cu–Pt ordering is widely observed in Ga0.5In0.5P layers grown by organometallic vapor phase epitaxy. The degree of order is a strong function of the input partial pressure of the phosphorus precursor, i.e., the V/III ratio, during growth. By observing the surface structure using in situ surface photoabsorption (SPA) measurements, the concentration of ‐oriented P dimers, characteristic of the (2×4) reconstructed surface, has been measured as a function of the growth conditions. For growth at 670 °C, the P‐dimer concentration is found to increase systematically as the input tertiarybutylphosphine pressure is increased from 10 to 200 Pa. This corresponds directly to a monotonic increase in the degree of order, measured using transmission electron microscopy and low‐temperature photoluminescence. These data strongly suggest that the (2×4) surface reconstruction is necessary for formation of the Cu–Pt structure. The step structure at the surface was also observed for these layers using atomic force microscopy. For high V/III ratios the structure of the layers grown on exactly (001) oriented GaAs substrates consists of islands surrounded mainly by bilayer (5.7 Å) steps. As the V/III ratio is reduced, the step height transforms to 2.8 Å (one monolayer). © 1996 American Institute of Physics.