This paper gives experimental vapor liquid equilibrium data for three binary systems: acetic acid-water,acetio acid-vinyl acetate,water-vinyl acetate,and for the ternary system: acetic acid-water-vinyl acetate in the region where liquid phases are mutually completely misoible.The activity coefficients corrected for vapor phase association of acetic acid are also calculated,thermodynamic consistency is tested,and the data are attempted to correlate with the NRTL equations.With the use of NRTL parameters of the three binary systems,the predicted.values of the ternary vapor liquid equilibria in the region where liquid phases arc completely miscible are consistent with the egperimental results.Experimental data and results of thermodynamic correlations of the ternary system in the region where liquid phases are partially miscible will be reported in Part Ⅱ.