The purpose of this work was to characterize the solid-state properties of anhydrous paclitaxel and paclitaxel dihydrate. Paclitaxel I (anhydrous) was suspended in water for 24 h to convert it to paclitaxel.2H2O. Both forms were analyzed by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). X-ray powder diffraction (XRPD) patterns were obtained at 25, 100, and 195 degrees C. Dissolution profiles of both forms were obtained in water at 37 degrees C over h. DSC of paclitaxel.2H2O showed two endothermic peaks below 100 degrees C, corresponding to dehydration. The resulting solid phase was termed "dehydrated paclitaxel.2H2O". At 168 degrees C, a solid-solid transition was observed in which dehydrated paclitaxel.2H2O was converted to a semicrystalline material called "paclitaxel I/am". The solid-solid transition was followed by melting at 220 degrees C. TGA of paclitaxel.2H2O showed a corresponding biphasic weight loss below 100 degrees C, which was equivalent to the weight of 2 mol of water. DSC of paclitaxel I showed no transitions before melting at 220 degrees C, and no weight loss was observed by TGA. Quenching of paclitaxel I from the melt produced amorphous paclitaxel with a glass transition at 152 degrees C. XRPD confirmed that paclitaxel I, paclitaxel.2H2O, and dehydrated paclitaxel.2H2O had different crystal structures. The X-ray patterns of paclitaxel I and paclitaxel I/am were similar, however the two forms of paclitaxel did not behave identically when analyzed by DSC. The bulk dissolution studies with paclitaxel I showed a rapid increase in concentration to 3 micrograms/mL in 4 h, which decreased to 1 microgram/mL after 12 h, corresponding to the solubility of paclitaxel.2H2O. The solubility of paclitaxel.2H2O was 1 microgram/mL. The data demonstrate the existence of a dihydrate form of paclitaxel that is the stable form in equilibrium with water at 37 degrees C but which dehydrates at temperatures > 45 degrees C.