In the operation of a tri-phase-separable type reaction process using polyethylene glycol (PEG) as a phase transfer catalyst, the effect of application of AC electric field on process behavior was investigated. First, dynamic behavior of the third phase under an AC electric field was analyzed for a phenyl benzoate production system. When AC voltage of high frequency was applied in a glass cell under operational conditions whereby the third phase was not separated after mixing, a whitish fluid was generated between the electrodes. Application of the AC electric field was seen to form a stable whitish layer near the interface between the aqueous and organic phases. Analysis of the dynamic behaviors of the whitish layer revealed that the rate of formation of the layer increased with the mole ratio of potassium hydroxide to phenol and the molecular weight of PEG. An electrothermal effect was considered to cause local phase separation of PEG in the vicinity of electrodes and free convection of the separated PEG phase, which resulted in acceleration of phase separation as a layer. Therefore, the effect of timing of AC voltage application on reactivity of third phase was investigated for operational conditions whereby the three phases were separated after mixing. We proposed a method of applying AC voltage at the moment when interface between the third phase and the aqueous phase was clearly visible after mixing. Experiments in a flow-type reactor demonstrated that the proposed method shortened the time interval between mixing and injection of reactant that should be set to achieve higher selectivity.