Stoichiometric ionization constants of acetic acid were determined in the temperature range (10 to 40) degrees C, from potentiometric titrations in aqueous potassium chloride solutions with molalities up to 3 mol.kg(-1). The electrochemical cell was calibrated in terms of hydrogen ion concentration, and the calculations were performed by means of the SUPERQUAD computer program. The Pitzer theory, widely used in the evaluation of the excess free energy of nonideal electrolyte solutions, was applied to the calculation of the activity coefficient of each chemical species involved in the equilibria, and a good fit was observed at all temperatures. From these results, Pitzer interaction coefficients for potassium acetate were determined. This model was also used to calculate the chloride ion activity coefficient in acetate buffer systems with added potassium chloride, at ionic strengths higher than 0.1 mol.kg(-1), the limit of validity of the Bates-Guggenheim convention.