The ionization constants (pKa) and the pH-dependent solubility (log S-pH) of six phenothiazine derivatives (promazine hydrochloride, chlorpromazine hydrochloride, triflupromazine hydrochloride, fluphenazine dihydrochloride, perphenazine free base, and trifluoperazine dihydrochloride) were determined at 25 and 37 °C. The pKa values of these low-soluble surface active molecules were determined by the cosolvent method (n-propanol/water at 37 °C and methanol/water at 25 °C). The log S-pH profiles were measured at 24 h incubation time in 0.15 M phosphate buffers. The log S-pH “shape-template” method, which critically depends on accurate pKa values (determined independently of solubility data), was used to propose speciation models, which were subsequently refined by rigorous mass-action weighted regression procedure described recently. Differential scanning calorimetry (DSC), UV–visible spectrophotometry, potentiometric, and high performance liquid chromatography (HPLC) measurements were used to characterize the compounds. The intrinsic solubility (S0) values of the three least-soluble drugs (chlorpromazine·HCl, triflupromazine·HCl, and trifluoperazine·2HCl) at 25 °C were 0.5, 1.1, and 2.7 μg/mL (resp.). These values increased to 5.5, 9.2, and 8.7 μg/mL (resp.) at the physiological temperature. The enthalpies of solution for the latter compounds were exceptionally high positive (endothermic) values (99–152 kJ·mol− 1). Cationic sub-micellar aggregates were evident (from the distortions in the log S-pH profiles) for chlorpromazine, fluphenazine, perphenazine, and trifluoperazine at 25 °C. The effects persisted at 37 °C for chlorpromazine and trifluoperazine. The solids in suspension were apparently amorphous in cases where the drugs were introduced as the chloride salts.