The esterification kinetics of acetic acid with methanol in the presence of hydrogen iodide as a homogeneous acid catalyst was studied with isothermal batch experiments at 30-60 degrees C. The catalyst concentration was varied between 0.05 and 10.0 wt%. The experiments revealed that besides the main reaction, the esterification of acetic acid, a side reaction appeared : the catalyst, hydrogen iodide, was esterified by methanol to methyl iodide. Plausible reaction mechanisms for methyl acetate and methyl iodide formation were proposed. The rate-determining step in the acetic acid esterification was assumed to be the nucleophilic attack of methanol to the carbenium ion formed through proton donation to acetic acid, whereas the rate-determining step in the hydrogen iodide esterification was presumed to be the substitution of the iodide to protonated methanol. Hydrogen iodide and acetic acid act as proton donors; thus the protolysis equilibria of acetic acid and hydrogen iodide were included in the mechanism. Rate equations, concentration-based as well as activity-based with UNIFAC activity coefficient estimations, were derived, and the kinetic and equilibrium parameters included in the rate equations were estimated from experimental data with regression analysis. Simulation of the models with the estimated parameters revealed that the rate equations predict correctly the experimental trends in the acid catalyzed esterification.