The pKa and log P of 20 compounds, including six substituted phenols, two substituted quinolines, N-methylaniline, five barbiturate derivatives, two phenothiazines, and several other molecules of pharmaceutical interest, were determined by the potentiometric technique at 25 degrees C and ionic strength 0.1 M (KNO3). The log P values were determined also by partition HPLC. Three of the substances were of very low aqueous solubility, and for these the aqueous pKas were determined by extrapolation from methanol-water solutions using the Yasuda-Shedlovsky technique. Values of log P obtained both by potentiometry and by partition HPLC, which ranged from 0.3 to 5.4, were in very good acordance with literature values. The general applicability of the potentiometric technique to ionizable compounds of diversely varied structures was demonstrated by the study.