Preparation and purification of fludarabine phosphate (I) from 9- beta -D-arabinofuranosyl-2-fluoroadenine, trimethyl phosphate and phosphorous oxychloride, comprises recrystallisation of the solid reaction product from deionised water in the presence of 20 - 200 wt.% acidic cation exchange material (with respect to (I)) and optionally activated carbon. The use of acidic ion exchange materials for the preparation and purification of (I) is also claimed.
