The surface potential technique is employed to estimate the dissociation constant (K) of a series of aromatic carboxylic acids in the form of Langmuir monolayers. Using electric surface potential (Delta V) measurements of monolayers spread on aqueous subphases, containing different concentrations of Nai, and applying the Gouy-Chapman theory as well as the Henderson-Hasselbalch equation, the surface pK for the investigated compounds falls in the range of 4.5-5.6 with the mean value of 5.1. The other method, based on surface potential measurements of monolayers of the parent, unsubstituted compound (5'-phenyl-m-terphenyl-4-carboxylic acid) spread on aqueous solutions with distinct pHs, leads to a pK of ca. 6.5 which corresponds to the bulk conditions. This value is shifted towards neutral pH as compared to the one calculated with the Henderson-Hasselbalch equation (5.3), owing to the inhomogeneous distribution of hydrogen ions caused by partial ionization of carboxylic groups of the monolayer molecules.