Laminar mixing of very viscous liquids and its effect on the course of parallel chemical reactions have been studied theoretically and experimentally. An existing model of laminar micromixing, based on an integral transformation of material balance equations in a local frame of reference attached to the Lagrangian point has been extended to account for the effects of differential diffusion. Smooth presumed algebraic functions were used to approximate concentration profiles of species in the local frame of reference and to derive ordinary differential equations for the concentration moments of the reactants. In experiments a solution of sodium hydroxide was mixed with a premixture of hydrochloric acid and ethyl chloroacetate in a co-rotating twin-screw extruder in a laminar flow regime. The viscosity of both solutions was increased by adding polyethylenepolypropylene glycol. In the experiments the effects of the screw speed, the extruder throughput, the volume ratio of the mixed reactant solutions and the degree of fill of the extruder on the selectivity of the parallel reactions: acid-base neutralisation and ester hydrolysis, were determined. The model was used to interpret the experimental results.