It is an important technique to obtain target products with higher yields by controlling the reaction and mass transfer. In this work, we focused on molecular weight control in the oxidative polymerization of phenol. We attempted selective production of phenol dimers through the formation of microdroplets in both single- and two-phase systems. In a single-phase system, selectivity of dimers to polymers can be improved by rapidly mixing ingredients using a micromixer. Further, we clarified the conditions to utilize such microspace effects by comparing the initial reaction time constant with the mixing time constant by order-of-magnitude estimation. In a two-phase system, it was clarified that controlling of mass transfer by formation of microdroplets using a micromixer was effective in the control of consecutive elementary reactions though the dimer selectivity hardly changes and the effect of micromixing was limited.