A pharmaceutical composition comprising (i) an amorphous solid solution comprising 9-[2-[bis[(pivaloyloxy)-methoxy]phosphinyl]methoxy]ethyl]adenine ("AD") and copovidone, which amorphous solid solution is prepared by dissolving AD and copovidone in a liquid organic solvent and evaporating the organic solvent, wherein the liquid organic solvent is acetone; and (ii) pharmaceutically acceptable inactive ingredients microcrystalline cellulose, silicon dioxide, magnesium stearate and anhydrous lactose, wherein the pharmaceutical composition is a solid dosage form, and wherein when the pharmaceutical composition is maintained at a temperature of 40 °C and at a relative humidity of 75% for 3 months, the AD degrades to provide a composition containing no more than 0.75% by weight of a less bioavailable impurity selected from 9-[2-(pivaloyloxy)-methoxyphosphinyl]methoxy]ethyl]adenine and adefovir relative to the original amount of AD, wherein the pharmaceutical composition is obtained by - dissolving AD in acetone along with copovidone; and - placing the solution in trays and drying rapidly in a vacuum oven operating at a temperature of no more than 35-40°C and at pressures close to 0 atm for 2-3 hours; and - removing the resulting dried film from the trays and milling it to a fine powder; - re-drying the fine powder in a regular convection oven for 4-6 hours at a temperature of not more than 35-40°C to remove any residual acetone; and - dry mixing the powder obtained after re-drying with said pharmaceutically acceptable inactive ingredients. (i) an amorphous solid solution comprising 9-[2-[bis[(pivaloyloxy)-methoxy]phosphinyl]methoxy]ethyl]adenine ("AD") and copovidone, which amorphous solid solution is prepared by dissolving AD and copovidone in a liquid organic solvent and evaporating the organic solvent, wherein the liquid organic solvent is acetone; and (ii) pharmaceutically acceptable inactive ingredients microcrystalline cellulose, silicon dioxide, magnesium stearate and anhydrous lactose, wherein the pharmaceutical composition is a solid dosage form, and wherein when the pharmaceutical composition is maintained at a temperature of 40 °C and at a relative humidity of 75% for 3 months, the AD degrades to provide a composition containing no more than 0.75% by weight of a less bioavailable impurity selected from 9-[2-(pivaloyloxy)-methoxyphosphinyl]methoxy]ethyl]adenine and adefovir relative to the original amount of AD, wherein the pharmaceutical composition is obtained by - dissolving AD in acetone along with copovidone; and - placing the solution in trays and drying rapidly in a vacuum oven operating at a temperature of no more than 35-40°C and at pressures close to 0 atm for 2-3 hours; and - removing the resulting dried film from the trays and milling it to a fine powder; - re-drying the fine powder in a regular convection oven for 4-6 hours at a temperature of not more than 35-40°C to remove any residual acetone; and - dry mixing the powder obtained after re-drying with said pharmaceutically acceptable inactive ingredients. - dissolving AD in acetone along with copovidone; and - placing the solution in trays and drying rapidly in a vacuum oven operating at a temperature of no more than 35-40°C and at pressures close to 0 atm for 2-3 hours; and - removing the resulting dried film from the trays and milling it to a fine powder; - re-drying the fine powder in a regular convection oven for 4-6 hours at a temperature of not more than 35-40°C to remove any residual acetone; and - dry mixing the powder obtained after re-drying with said pharmaceutically acceptable inactive ingredients.