An apparatus based on the inert gas stripping method was assembled for measuring infinite dilution activity coefficients, gamma(infinity), of 15 apple juice aroma compounds in aqueous solutions. Values were estimated using the UNIFAC model and compared with experimental results; differences ranged from -80 to 118% with an average of 48% for the absolute values of the differences. Values predicted with recent UNIFAC parameters were in better agreement with experimental results than those obtained using original parameters. The effect of temperature on gamma(infinity) and relative volatility of ethyl acetate and acetone was also studied. Measurements were conducted for esters in solutions containing different levels of ethanol. A decrease in gamma(infinity) with increasing ethanol concentrations was observed for esters in solutions containing ethanol, especially at mole fractions > 10(-2).