According to a recent study, the compositional nonuniformity for group V sites of quaternary (Q) In0.68Ga0.32As0.7P0.3 layers (λ=1.35 μm) grown by low‐pressure metalorganic vapor phase epitaxy (LP‐MOVPE) on 50 mm diam InP wafers was reduced from 3% to 1% when tertiary butyl phosphine (TBP)+AsH3 instead of PH3+AsH3 or TBP+tertiary butyl arsine (TBAs) were employed. In this letter we offer a thermodynamic interpretation of these observations. Under LP‐MOVPE conditions at a total pressure of 0.08 atm for fully decomposed group V precursors diluted with H2, we show that while the dominant species are the tetramers, the concentration of the dimers is significant and becomes more prominent with rising temperature (T). Examining the [As/P] ratio in the gas phase as a function of reciprocal T, we demonstrate that in the MOVPE range (900–950 K) the slope of these curves changes from steep to moderate and then nearly flat for the sources AsH3+PH3, TBAs+TBP, and AsH3+TBP, respectively. The insensitivity of the [As/P] ratio to the ∼30 K gradient over the susceptor when combining AsH3 and TBP suggests a very effective method to assure compositional uniformity in Q layers prepared by LP‐MOVPE. Uniformity results for atmospheric pressure MOVPE employing AsH3 and PH3 are also consistent with the thermodynamic argument.