In the reactions usedto break heparin downto mono-and oligosaccharides, anhydrosugars are formed at two stages. The first of these is the well-known cleavage of heparin with nitrous acid to convert the N-sulfated D-glucosamines to anhydro-D-mannose residues; this reaction has been studied in detail. It is demonstrated here that only low pH (< 2.5) re-action conditions favor the deamination of N-sulfated D-glu-cosamine residues; the reaction proceeds very slowly at pH 3.5 or above. On the other hand, N-unsubstituted amino sugars are deaminated at a maximum rate at pH 4 with markedly reduced rates at pH 2 or pH 6. At room temperature solutions of nitrous acid lose one-fourth to one-third of their capacity