This work studied the liquid phase of acetic acid and butyl alcohol esterification reaction (Patm = 560 mmHg), using an ion exchange resin (Lewatit K-2431) as catalyst. A set of assays were carried out for determining the effect of catalyst load, temperature and molar ratio (acid/alcohol) on chemical equilibrium constant. Components' selective sorption on the resin matrix was noticed; its effect on equilibrium conditions was verified, by using different acid/alcohol starting ratios. A non-ideal model (UNIQUAC) was used for calculating component activities. Standard free energy and enthalpy of esterification were calculated (∆G° = -1.874 Kcal/mol, ∆H° = -5.335 Kcal/ mol), corresponding to reported data from previous work. An expression for calculating apparent equilibrium constant was obtained, considering thermal effects and those associated with liquid-polymer phase equilibrium. Such expression had good agreement with experimental and reported data.