Disclosed is anhydrous bosentan polymorphic Form B, characterised by having an XRPD pattern comprising peaks at 9.6, 16.1, 17.1, 18.4 and 5 21.8 °2q + 0.2°2q. Further disclosed is a process for preparing the above anhydrous polymorphic form which comprises: adding bosentan to a mixture of ethyl acetate and heptane, heating the mixture to the reflux temperature of the solvent mixture, cooling the mixture to a temperature ranging from 20°C to 30°C, and drying the resulting solid at a temperature above 60°C to obtain anhydrous bosentan Form B.
