Predictions of flow fields in a stirred tank reactor, obtained by computational fluid dynamics, were used for the simulation of a mixing sensitive process consisting of two parallel reactions competing for a common reagent: A + B --> Prod.1 A + C --> Prod.2. Experimental data were obtained for A = OH-, B = 1/2Cu(++) and C = ethyl-chloroacetate. For this reaction scheme the final selectivity of the process, easily measured by a simple colorimetric analysis of the residual Cu++, was found to depend on agitation speed and therefore on the mixing history during the batch process. The flow field-based three-dimensional simulations performed here led to predictions that compared very well with the experimental data, though no adjustable parameters were used. Interestingly, these encouraging results were obtained by modelling only the "macromixing" phenomenon, while "micromixing" phenomena were neglected, i.e. the system was always considered as being locally perfectly micro-mixed. The good agreement found between simulation predictions and experimental data retrospectively confirms the negligibility of micromixing phenomena in the system investigated.