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Ga0.5In0.5P layers have been grown by organometallic vapor-phase epitaxy using various values of input V/III ratio for two phosphorus precursors, phosphine, the conventional precursor, and tertiarybutylphosphine (TBP), a newly developed, less-hazardous precursor. For growth on nominally (001) GaAs substrates misoriented by 3° (and in some cases by 0° or 6°) to produce [110] steps on the surface at a growth temperature of 620 °C, the Cu–Pt-type ordering is found to be strongly affected by the input flow rate of the phosphorus precursor (V/III ratio). For decreasing input partial pressures below 3 Torr for PH3 and 0.75 Torr for TBP the low-temperature photoluminescence (PL) peak energy increases indicating a lower degree of order. This is confirmed by transmission electron diffraction results. The decrease in the degree of order corresponds to a decrease in the concentration of [1̄10]-oriented P dimers on the surface, as indicated by surface photoabsorption spectroscopy results. These data indicate that the reduction in ordering is caused by the loss of the (2×4) reconstructed surface during growth. The difference in the behavior for PH3 and TBP is interpreted as due to the lower pyrolysis efficiency of PH3. The surface structure measured using high-resolution atomic force microscopy indicates that the [110] steps produced by the intentional misorientation of the substrate are bunched to produce supersteps approximately 30–40 Å in height for the lowest V/III ratios. The step height decreases markedly as the input phosphorus partial pressure increases from 0.4 to 0.75 Torr for TBP and from 1 to 3 Torr for PH3. This corresponds to a change from mainly monolayer to predominantly bilayer steps in the vicinal regions between bunched supersteps. Stabilization of the bilayer steps is interpreted as due to formation of the (2×2) reconstruction on the (111)B step edges. The degree of order is an inverted U-shaped function of the flow rate of the phosphorus precursor. Thus, use of very high input V/III ratios is also found to reduce the degree of order in the Ga0.5In0.5P layers. These high input phosphorus flow rates are found to result in a monotonic increase in the density of [1̄10]-oriented P dimers on the surface. This decrease in order is believed to be related to a change in the structure of kinks on the [110] steps at high V/III ratios.