Didanosine is an antiviral drug. During the preparation of didanosine in our lab, six process related known impurities and one unknown impurity were detected in HPLC analysis at levels ranging from 0.05 to 0.8%. The same unknown impurity was also observed in commercial batches. This new impurity was isolated by preparative HPLC and co-injected with didanosine sample to confirm the retention times in HPLC. This impurity was characterized as, 9-(2,3,5-trideoxy-beta-D-glycero-pentofuranosyl)-9H-purin-6-one (2',3',5'-trideoxyinosine). Structural elucidation of this impurity by spectral data (1H NMR, 13C NMR, MS and IR) has been discussed.