The reaction kinetics of the reversible liquid-phase esterification of acetic acid with 1-butanol was investigated at temperatures between 353.1.5 and 393.15 K using sulfuric acid as catalyst. The main goal of the work was to Study the influence of the initial composition on the reaction rate constants of that industrially important reaction. It is shown that the reaction rate is faster in butyl acetate-rich mixtures than in water-rich mixtures. The experiments were carried out in a batch reactor using quantitative online H-1 NMR spectroscopy for analysis. For describing the results, a second-order reaction kinetic model in activities or fugacities, respectively, is used, which is consistent with a recently developed thermodynamic model of phase and reaction equilibrium. Different thermodynamic models are compared: NRTL, UNIQUAC, and PC-SAFT. All of them can successfully correlate the kinetic data, but none allows a prediction of the concentration dependence of the reaction rate constants. The results show that introducing activities of the reacting components alone is not sufficient for predicting the concentration dependence of the reaction rate constants as the composition of the reacting Mixture has a strong influence on the activity of the catalyst itself.