The objective of this study was to reduce the crystallinity of ursodeoxycholic acid (UDCA) by solid dispersion with controlled pore glass (CPG). To evaluate the effect of pore diameter and pore volume of CPG on the crystalline properties of UDCA, we used powder X-ray diffractometry (PXRD) and differential scanning calorimetry (DSC). PXRD patterns and the DSC data indicated the presence of UDCA in a crystalline state in the physical mixtures. It was found that amorphous UDCA could be formed via solid dispersion with CPG obtained by a solvent method. The DSC thermograms of solid dispersions showed that there were two states of UDCA, amorphous and crystalline. The amount of crystalline fraction in the solid dispersions depended on the pore size, pore volume, and the specific surface area of CPG. When UDCA was mixed with different pore diameters of CPG, it was found that UDCA molecules preferentially interacted with pores of smaller size. Copyright 1999 Academic Press.