Mixing has a major influence on the product ratio of fast competitive reactions, as the product ratio of these reactions is determined by local concentrations. To integrate models from literature for the description of the mean and fluctuating concentration of a continuous reactant flow fed to a stirred-tank reactor, the models are validated against detailed experimental data for a stirred-tank reactor equipped with a Rushton turbine impeller. Application of these models requires information on local hydrodynamic parameters, which are determined by laser Doppler velocimetry. Model calculations are validated by determining the mean concentration and concentration variance of an inert tracer fed with planar laser-induced fluorescence to the studied stirred-tank reactor. To calculate the mean concentration, a combination of a theoretical model, measured mean concentrations and LDV measurements is used to determine the turbulent diffusion coefficient. The turbulent mixer model is used to calculate the concentration variance. Compared to the literature, this model requires adjustment of the constant in the production of the concentration variance to fit the measured concentration variances correctly. For the mean concentration and its variance, measurements agreed reasonably with simulations.