5-Chlorouridine (III), 5-chlorocytidine (IV), and 5-chloro-1-β-D-arabinofuranosylcytosine (VIII) were obtained in a yield of 61%, 61%, and 11.2% respectively by reaction of pyrimidine nucleosides with N-chlorosuccinimide. 2, 2'-Anhydro-5-chloro-1-β-D-arabinofuranosyl cytosine (XI) was prepared in a yield of 53% by reaction of IV with Vilsmeier-Haack reagent. 5-Chloropyrimidine nucleosides (III, 5-chloro-1-β-D-arabinofuranosyluracil (VII), IV, and VIII) were found to degrade in aqueous alkali (1 N NaOH) at 50° via deamination or 6, 2' (or 5')-anhydro open-chain ureido compounds. IV was more readily degraded in 0.3 N KOH at 37° than non-chlorinated (II). An unique reaction was that the treatment of XI with 0.1 N NaOH at room temperature afforded 2, 2'-anhydro-5-hydroxy-1-β-D-arabinofuranosylcytosine (XII) without splitting the anhydro bond.