In this paper two different gas inlet modifications for a horizontal single-wafer MOVPE reactor were studied: the conventional one ensured gas mixing of groups III and V precursors by forcibly merging both gases in a gas mixing unit, the newer one allowed mixing of groups III and V precursors solely by diffusion after the separation plate. AlAs/ GaAs Bragg reflector structures were deposited in nitrogen atmosphere for both gas inlet types and the uniformity determined by X-ray diffraction. Three-dimensional detailed numerical modeling was employed to explain the results. They establish that the observed strong non-uniformity of growth for the new inlet as well as the unusual concave instead of the common convex growth profile found is due to the incomplete mixing of precursor gases in a highly dense gas phase. By adjusting the groups III/V gas stream ratio an effect could be taken on the diffusion processes. Furthermore the total flow in the reactor was optimized for the new gas inlet. By taking advantage of changed diffusion properties in nitrogen atmosphere, however, perfect uniformity with an absolute deviation smaller than 1% was obtained. The modeling results are presented and agree very well with the experimental data.