As the first part of a series of studies aimed at synthesizing 2-phenoxyisobutyric acid ethyl ester by tri-liquid-phase catalysis, this work investigates the factors influencing the formation of a third liquid phase. In this tri-liquid-phase system, without adding a hard base, 2-bromoisobutyric acid ethyl ester is used as an organic substrate, sodium phenolate (NaOPh) as an aqueous nucleophilic reagent, and tetra-n-butylammonium bromide (QBr) as a phase-transfer catalyst. The factors influencing the formation include the total amount of NaOPh and QBr added, mole fractions of NaOPh and QBr, kinds of organic solvent, kinds and amount of salt, and operating temperatures. Experimental results indicate that in addition to classifying changes in the volume and Q(+) concentration of the third liquid phase into three categories according to the mole fraction of QBr, such changes are rationalized by the solubilities of the products of the reaction among NaOPh, QBr, and water and the complexes subsequently formed. Moreover, the volume of the third liquid phase varies when adding the organic reactant, owing to the change in polarity of the organic phase. Furthermore, adding a salt (NaBr, KBr, or Na2CO3) or a high temperature enhances the formation of a third liquid phase. When an appropriate amount of NaBr is added to the system containing 5 mL of n-heptane and 5 mt of water, a total of 10-15 mmol of NaOPh and QBr, with a mole fraction of QBr between 0.3 and 0.5 (RBr was absent), might give an optimal volume and Q(+) concentration of the third liquid phase for the nucleophilic substitution reaction with NaOPh as an aqueous reactant by the tri-liquid-phase catalysis regardless of what kinds of organic reagent are used.