The product concentration profile that develops as a result of an instantaneous bimolecular reaction when contacting semi-infinite strained reactant solution films, is modelled and analysed. Analytical solutions to reactant and product concentrations are presented for which initial reactant concentrations, reactant diffusivities, and product diffusivity can be independently and arbitrarily chosen. The validity of the model for finite geometry is thoroughly discussed. It is shown that the product concentration profile may differ significantly depending on diffusivities and reactant concentrations, even for cases when almost the same amount of product is produced. The maximum product concentration is constant in time but varies with diffusivities and reactant concentrations in a quite complicated fashion. Under certain conditions the maximum product concentration may even decrease when initial reactant concentrations or reactant diffusivities are increased. Dimensionless concentration profiles and the maximum concentration are invariant with mixing intensity. The influence of mixing is fully incorporated into the length and time scales. The actual real time to form a certain amount of product is inversely proportional to the square root of the specific local energy dissipation rate. The implications of model results on fast complex reactions are discussed.