[en] Numerical calculations are described that model the flow, diffusive and reactive processes that occur in a typical flowing afterglow apparatus. These calculations differ from previous solutions to the problem in that the changes in the ambipolar diffusion coefficients that occur with changes in ion composition are considered explicitly. It is shown that these changes can cause distinct curvature in plots of log(ion concentration) versus neutral reactant flow. The effects of axial diffussion and reactant inlet geometry are also considered explicitly. The results are compared with recent experimental data on ion/molecule reactions between UF₆ and various negative ions. (orig.)