The effect of basic process conditions and the initial distribution of mixed liquids on mixing on the molecular scale was studied experimentally for helical and twisted bends reactors. The selectivity of two competitive-parallel reactions, acid-base neutralization and alkaline ester hydrolysis, carried out in aqueous solutions between initially unmixed reactants, was used to quantify micro mixing in steady-state, isothermal and laminar flow. Faster micromixing is achieved in the twisted bends mixer than in the helical mixer due to the periodic reorientation of Dean vortices, present in curved conduits. The difference in micromixing performance of the two reactors decreases for high Reynolds numbers (Re > 1000) when ester hydrolysis is no longer controlled by mixing. Micromixing for lower Reynolds numbers can be improved by changing the ratio of volumetric flows of the reactant solutions and by using a distribution head, which at the reactor entrance creates the contact surface between mixed liquids perpendicular to the curvature plane of the first pipe bend. When micromixing in the reactor becomes so slow that the final selectivity approaches its maximum value, changing the ester to a slower reacting one helps to adjust the sensitivity of the test reaction system to the deteriorating mixing conditions. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.