Hydrophobicity values (log P-o/w) determined both by potentiometry and by the conventional shake-flask method and aqueous pK(a) values obtained by potentiometry for seven isoniazid and nine benzimidazole derivatives are presented. Several of these compounds have strong hydrophilic or hydrophobic character, and some of them show two or more ionizable sites, sometimes very acidic or very basic. These facts have compelled us to work near the limits of the potentiometric technique, and therefore, a critical evaluation of the results obtained in these extreme working conditions is presented. Particularly, details about the potentiometric determination of log P-o/w values of the most hydrophilic and most hydrophobic compounds are clearly described.