The present invention is directed to a process for the preparation of Efavirenz, (4S)-6-chloro-4-(cyclopropylethynyl)-4-(trifluoromethyl)-1,4-dihydro-2H-3,1-benzoxazin-2-one, of formula (I) comprising reacting the intermediate of formula (II) [0045] as a free base or a salt thereof, with diphosgene (TCMCF, trichlormethylchloroformate) Cl3CO—COCl in an organic solvent or in a biphasic medium comprised of an organic solvent and water, preferably but not mandatorily in the presence of a weak base in an amount sufficient to neutralise the reaction mixture or in an up to 30% molar excess of such amount.