The experimental tie-line data measured for two quaternary mixtures of the water- ethanol-2,2,4-trimethylpentane-methyl tert-butyl ether (MTBE) and water-ethanol-2,2,4-trimethylpentane-tert-amyl methyl ether (TAME) systems at the temperature 298.15 K and ambient pressure are presented. The experimental quaternary liquid-liquid equilibrium data have been satisfactorily correlated by using a modified form of the UNIQUAC model with binary, ternary and quaternary parameters. The calculated results are further compared with those obtained from the extended UNIQUAC model of Nagata.