We have developed direct esterification reaction of carboxylic acids with alcohols catalyzed by diarylammonium salt catalysts toward the recovery of acetic acid from wastewater of chemical plants. Hydrophobicity of the catalysts could suppress their solubility in water. This esterification reaction has been applied to the recovery of acetic acid using a flow reaction system which is applicable to practical use in chemical plants. It was shown that this catalytic system was also applicable to the combination of hydrophobic substrates in water. (C) 2012 Elsevier B.V. All rights reserved.