The complete phase diagram for the poly(ethylene glycol) dimethyl ether 2000 (PEGDME(2000)) + Na(2)CO(3) + H(2)O system at 298.15 K was determined. Experimental liquid-liquid equilibrium phase diagrams, tie lines, and plait points were obtained for the ternary system. Compositions of the liquid-liquid and the liquid-liquid-solid equilibria were determined from calibration curves of refractive index of the solutions, and atomic absorption (AA) and X-ray diffraction analyses were made on the solids. Binodal curves were described using the Merchuk equation at T = (288.15, 293.15, 298.15, 308.15, and 318.15) K, and the fitting parameters were obtained for the corresponding temperatures. The effects of temperature on the binodal curve were also studied, and it was observed that the area of the biphasic region increased slightly with increase in temperature. Also, the tie lines were fitted to both the Othmer-Tobias and Bancroft and Setschenow-type equations. Correlation coefficients for all equations are reported.