The present invention relates to an improved process for preparing [(1S,2R)-3-[[(4-aminophenyl)-sulfonyl](2-methyl-propyl)amino]-2-hydroxy-1-(phenylmethyl)-propyl]-carbamic acid (3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-yl ester—which compound is also known under its INN as darunavir—by reacting carbonic acid 2,5-dioxo-1-pyrrolidinyl[(3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-yl]ester with 4-amino-N-[(2R,3S)-3-amino-2-hydroxy-4-phenylbutyl]-N-(2-methylpropyl)-benzenesulfonamide in ethanol as solvent. Furthermore said process allows for darunavir to be isolated immediately in its ethanolate form, i.e. darunavir monoethanolate, which is the marketed form of darunavir under the tradename Prezista™.