This study aims to measure the solubilities of poorly water-soluble drugs and to enhance their solubilities by adding co-solvents. The solubilities were measured using high-performance liquid chromatography (HPLC). We selected famotidine, a histamine H-2-receptor antagonist, as the pharmaceutical compound. We measured the solubilities of famotidine at 298.15K in five water/co-solvent mixed solvents, that is, two water + liquid co-solvent mixtures: water + ethanol, and water + polyethylene glycol (PEG) 400; and three water + solid co-solvent mixtures: water + beta-cyclodextrin (beta-CD), water + PEG 1000, and water + lauryl sulfate (SLS). In the two water + liquid co-solvent mixed solvents, the solubilities were measured over the entire co-solvent mole fraction range. In the three water + solid co-solvent mixtures, the solubility was determined up to the co-solvent saturation point. In addition, for the three solid co-solvents, i.e. beta-CD, PEG 1000, and SLS, the solubilization power of each co-solvent-solute system was evaluated using a log-linear model. The experimental solubility data for the two water + liquid co-solvent mixtures were correlated using three local composition models: modified Wilson, NRTL 1, and UNIQUAC. For the three water + solid co-solvent mixtures, a modified Chrastil model was used to correlate the experimental solubility data. (C) 2010 Elsevier B.V. All rights reserved.