The　data　of　chemical　and　phase　equilibrium　(CPE)　were　measured　for　the　quaternary　system　methyl　acetate　(1)-methanol(2)-water(3)-acetic　acid(4)　with　a　modified　Rose　Williams　equilibrium　still　under　0.1013MPa.　Wilson,　NRTL　and　UNIQUAC　equations　were　used　to　calculate　active　coefficient　in　the　liquid　phase.　The　″chemical　theory″　was　used　to　correct　the　nonideality　of　the　vapor　phase　with　associated　molecules.　The　predicted　results　of　CPE　for　the　quaternary　system　show　good　agreement　with　experimental　data.