LLE　(liquid-liquid　equilibrium)　data　have　been　determined　in　the　LLE　still　for　the　quaternary　system　of　methyl　acetate-methanol-water-acetic　acid　at　20degreesC,30degreesC,40degreesC　and　50degreesC　under　atmospheric　pressure.　These　works　can　provide　sufficient　data　support　for　industry　design　and　add　new　LLE　data　into　the　data　bank.　According　to　the　model　parameters　correlated　by　the　activity　coefficient　models　including　NRTL　and　UNIQUAC,　LLE　data　of　the　quaternary　system　were　predicted　by　simplex　optimization　and　the　comparison　between　calculated　and　experimental　data　shows　good　agreement.