In this study, the retention behavior of benzimidazole anthelmintic drug mebendazole (MBZ) was investigated with a liquid chromatographic method. Influence of acetonitrile (ACN) concentration present in the mobile phase on retention measurement was examined. Analyses were carried out with a Kinetex Core-Shell C8 (Phenomenex, 150 x 4.6 mm I.D., 2.6 mu;m) column at 25 degrees C. The mobile phase pH ((s)(s)pH) values were measured in ACN + water mixtures. By using the retention data obtained from this chromatographic analysis, thermodynamic dissociation constant (pK(a)) values of the mebendazole were investigated experimentally at a constant flow rate. Experimental data were analyzed by using a nonlinear regression program. This is the first time that the pH-retention time relationships of MBZ were evaluated by the experimental method. Moreover, the pK(a) values at 37 degrees C, solubility (log S-w), and lipophilicity (log P-o/w) were calculated by using Abrahams solute descriptors. The pK(a) values obtained from this study complied with the literature data acquired from other methods.