This contribution emphasises physically based models for the mixing-reaction coupling. The fluid mechanical information needed to implement such models can then be identified. A semi-batch stirred tank reactor carrying out the competitive-consecutive reactions A + 8 = R; R + B = S is considered. Reagent A is in the tank and B is added so slowly that it mixes with the tank＇s average contents. Only micromixing then determines the product distribution between R and S, whereby segregation promotes S. The second topic is micromixing when the concentrations in the surroudings of the reaction zones are determined by mesomixing i.e. mixing which is intermediate in scale between that of micromixing and that of the whole vessel. Fresh B, emerging from a feed pipe, is transported by the local velocity field and dispersed transverse to its local streamline by turbulent diffusion. Dispersion gives an intermingling of A (from surroundings) and B (from feed pipe) which is coarse relative to the dissipation scales.