Ternary liquid-liquid equilibrium (LLE) is presented for propylene oxide + water + propylene dichloride at 27 degrees C. Activity coefficients of the three components are correlated with the Margules three-suffix equation. The parameters of the equation are evaluated by the Nelder-Mead optimization procedures using two different objective functions. LLE predictions are found to be better using the optimized parameters rather than those obtained from independent binary data.