The acid dissociation constants of D-glucosamine (I) and its three O-methyl derivatives were determined by titration method. The stability of I was also studied representing these compounds by the improved Fehling method preceding to the pH titration, and I was found to be stable in aqueous solution under experimental conditions. From the analysis of the titration data, I could be verified to behave as a mono basic acid by the comparison of the experimental titration curve with the one theoretically calculated. Finally, the pKa values of these four compounds were calculated.