Disclosed are methods of separating the diastereoisomers of gemcitabine-[phenyl-(benzoxy-L-alaninyl)]-phosphate (NUC-1031), or salts thereof, using crystallisation. In particular, crystallisation from isopropyl alcohol provides gemcitabine-[phenyl-(benzoxy-L-alaninyl)]-(S)-phosphate in high diastereoisomeric purity: Also disclosed is a crystalline form of gemcitabine-[phenyl-(benzoxy-L-alaninyl)]-(S)-phosphate. Disclosed are methods of isolating gemcitabine-[phenyl-(benzoxy-L-alaninyl)]-(R)-phosphate in high diastereoisomeric purity: