The phase diagram was determined for the Na2CO3-PEG-H2O system at 25 degreesC using PEG (poly(ethylene glycol)) with a molecular weight of 4000. Compositions of the liquid-liquid and the liquid-liquid-solid equilibria were determined using calibration curves of density and index of refraction of the solutions, and atomic absorption (AA) and X-ray diffraction analyses were made on the solids. The solid phase in equilibrium with the biphasic region was Na2CO3.H2O. Binodal curves were described using a three-parameter equation. Tie lines were described using the Othmer-Tobias and Bancroft correlation's. Correlation coefficients for all equations exceeded 0.99. The effects of temperature (25 and 40 degreesC) and the molecular weight of the PEG (2000, 3000, and 1000) on the binodal curve were also studied, and it was observed that the size of the biphasic region increased slightly with an increase in these variables,