In order to enhance the bioavailability of a poorly water-soluble drug by improving its solubility and dissolution rate, solution-enhanced dispersion by supercritical CO2 (SEDS) was used to nanonize methotrexate (MIX). We designed a 2(3) factorial experiment to investigate and identify the relative significance of the operating parameters on the surface morphology and mean size of MTX particles. The effect of the nonsolvent/solvent (acetone/dimethyl sulfoxide, v/v) ratio was found to exert a dominant influence on mean particle size. Increasing the nonsolvent/solvent ratio decreased the particle size significantly. Under the following conditions: nonsolvent/solvent ratio 6:1. MIX concentration 0.3%, flow rate 1.0 ml min(-1), temperature 308 K, and pressure 12 Mpa, the resulting MTX nanoparticles exhibit a good spherical shape, a smooth surface, and a narrow particle size distribution, with a mean particle size of 81 nm. After supercritical processing, there was no change in the chemical composition of the MTX. However, the physical state of the MIX shifted from crystalline into amorphous due to the minor structural changes which occurred on a molecular level. Due to the reduced particle size and change of physical state, the solubility and dissolution rate of the MIX nanoparticles were much higher than those of the original MIX powder (5.9 mu g ml(-1) versus 3.7 mu g ml(-1) in 42 h, and 12.6 mu g ml(-1) versus 10.8 mu g ml(-1) in 92 h). This study reveals that supercritical processing may have a great potential in improving the bioavailability of poorly water-soluble drugs. (C) 2012 Elsevier B.V. All rights reserved.