As the first part of a series of studies aimed at synthesizing n-butyl phenyl ether from sodium phenolate (NaOPh) and n-butyl bromide by tri-liquid-phase catalysis (TLPC), with poly(ethylene glycol) (PEG) as the catalyst, this work investigates the formation of a third liquid phase in the organic phase/PEG-600/aqueous (NaOPh + NaOH) system. The factors that influence the formation of this third phase include the total amount NaOPh and PEG-BOG added, mole fractions of NaOPh and PEG-BOG, kinds of organic solvents, kinds and amount of salt, molecular weight of PEG, and operating temperatures. Experimental results indicate that in addition to classifying the change in the minimum added quantity of NaOH on the formation of a third liquid phase into three categories according to the mole fraction of PEG-600, such a change is rationalized by the formation of PEG-600/Na+ and hydrated ions. Moreover, adding a salt (NaBr, KBr, NaI, or Na2CO3) to the system, using PEG-600 as a phase-transfer catalyst, would not generate a third liquid phase unless a hard base (e.g., NaOH) is added. Furthermore, high temperatures enhance the formation of a third liquid phase. Adding 0.6 g of NaOH to the system containing 5 mL of n-heptane, 5 mt of water, and a total of 0.010-0.015 mol of NaOPh and PEG-600, with a mole fraction of PEG-600 between 0.4 and 0.5, might produce an appropriate volume of the third liquid phase for the nucleophilic substition reaction with NaOPh as an aqueous TLPC reactant.