Two competitive-parallel chemical reactions proceeding between HCl, NaOH and CH2ClCOOC2H5 in water-polyethylenepolypropylene glycol systems are applied to study micromixing of liquids of different viscosities in a reactor with the Couette flow. A new micromixing model is proposed to interpret experimental results. Additionally a full set of physicochemical data characterizing the test reaction system is presented.