Systems composed of poly(ethylene)glycol (PEG 400) + water + either potassium carbonate or sodium carbonate were studied at 283.15, 298.15, and 313.15 K. The effects of temperature and the electrolyte on phase segregation were evaluated. The phase segregation process was endothermic and entropically driven. The efficacy of the cations in inducing phase formation with PEG 400 followed the order K2CO3 < Na2CO3. The binodal curves were successfully described using the empirical equation suggested by Merchuk and modified to include the effect of temperature. Tie-line compositions were correlated using the Othmer-Tobias, Bancroft, and Hand equations. The experimental tie-line data for PEG 400 + K2CO3 + water and PEG 400 + Na2CO3 + water were also correlated using the nonrandom two liquid model. The use of this thermodynamic model resulted in reliable data for the system while reducing the number of experiments needed. All of the correlations indicated satisfactory fits between the calculated and experimental data.