The effect of solid dispersion (SD) formation on the solubility of phenazepam has been studied. Phenazepam and its SDs with poly(ethyleneglycol)-1500 (PEG), poly(vinylpyrrolidone)-10000 (PVP), and β-cyclodextrin were studied. The SD with PVP increases both the solubility and the dissolution rate of phenazepam. Results obtained by a complex of physical and chemical methods suggest that the improved release of phenazepam from the SD with PVP is due to solubilization, amorphization, and formation of a colloidal dispersion of the parent drug substance.