We established two good linear relationships between pK(a) and acidic atom's electrostatic potential (MEP) in the gas phase by DFT methods at B3LYP/6-31+G(d, p) level of theory. We studied 13 experimentally well-characterized singly substituted benzoic acids to determine the best level of theory. A good linear relationship between MEP and the experimental pKa data has been revealed (standard deviation (STD) = 0.081, R-2 = 0.937 for model 1; STD = 0.081, R-2 = 0.934 for model 2), better than the computational results in the literatures. Our models also showed that it is reliable to estimate the pK(a) values of multiply substituted benzoic acids. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.