Semibatch stirred tank reactors with a turbulent flow field are used frequently in the chemical process industry to accomplish mixing tasks. Mixing in a turbulent flow consists of several processes. Descriptions for these processes from literature are used to construct a model to calculate the selectivity of a mixing-sensitive reaction in a semibatch stirred tank reactor. The model is validated by determining the effects of various process parameters on the selectivity of the third Bourne reaction. Calculation of the selectivity requires information on the hydrodynamic parameters of the stirred vessel studied. The essential detailed description of these hydrodynamic parameters to obtain a good agreement between measured and calculated selectivities was determined The model proved to be successful in predicting the product distribution of a competitive reaction, making it useful for the design and scale-up of stirred-tank reactors.