A systematic study is performed on the dependence of the Al-mole fraction in AlxGa1-xN on the organometallic group-III precursor flow during metal-organic vapor-phase epitaxy. When keeping the total organometallic volume flow constant, a nonlinear concave bowing relationship is found between the Al-mole fraction (for 0≤x≤1) and the relative trimethylaluminum volume flow. A kinetic model, which takes into account the growth rate ratio between GaN and AlN (gGaN/gAlN), is developed to explain such concave bowing relationship. The experimental data are in excellent agreement with the theoretical model. For AlxGa1-xN growth on AlN bulk substrates, it is found that the Al-mole fraction is smaller and the growth rate ratio is larger than on sapphire substrates. The authors also investigate the incorporation of Al in AlxGa1-xN as a function of the group-III precursor molar flow rate. A positive convex bowing relationship is found between Al-mole fraction and the relative trimethylaluminum molar flow, which is consistent with our model.