Density functional theory calculations have been performed on a complete series of hydrogen-bonded complexes of hydroxybenzoic acids with ammonia. The gas-phase O-H bond length and stretching frequency of the carboxylic group and the hydrogen-bond length and energy with ammonia are shown to be highly correlated with one another and are linearly correlated with the experimental aqueous pK(a) values of the hydroxybenzoic acids. The predicted pK(a) values from the different molecular properties, and are in good agreement with available experimental values. This study suggests that calculated molecular properties of hydrogen-bonded complexes allow effective and systematic prediction of pK(a) values. (c) 2009 Elsevier B.V. All rights reserved.