Surface behavior and bulk properties of two phenothiazine drugs, fluphenazine and trifluoperazine dihydrochlorides, are reported. Surface activity studies for both amphiphilic drugs were carried out in aqueous solutions at 20 degrees C by surface tension measurements. The volumetric and compressibility properties of the drugs were derived from density and ultrasound velocity measurements. Critical concentrations were determined by surface tension and ultrasound data. The experiments were realized in aqueous and buffered (pH 3.0, 5.5, and 9.2) media at 20 degrees C. The buffered media were chosen below and above the different pK(a)'s that the drug molecules present (pK(a1) and pK(a2) of 3.90 and 8.10, respectively).