A product distribution of a system of two parallel chemical reactions, consisting of acid-base neutralization and ester hydrolysis, was used to characterize mixing in a corotating twin-screw extruder. The experimental system was supplied through different feeding ports with the acid-ester premixture and the base solution respectively. A model of laminar micromixing was applied to correlate the screw speed and the extruder throughput with the average rate of viscous deformation for three screw geometries.