The ring-opening multibranching polymerizations of 1,6-anhydro-β-D-glucopyranose (1) and 1,6-anhydro-β-D-galactopyranose (2) have been studied in order to synthesize hyperbranched polysaccharides. The solution polymerization in propylene carbonate and the bulk polymerization of 1 and 2 using a thermally induced cationic initiator proceeded through a ring-opening reaction and a proton transfer reaction to afford highly water-soluble polysaccharides, i.e., poly-1 and poly-2, respectively. For the polymers from 1 and 2 with the same polymerization conditions, the M-w,M-SLS and yield of poly-1 were higher than those of poly-2. Here, poly-1 and poly-2 were characterized as hyperbranched polysaccharides consisting of α- and β-linked D-hexopyranosyl and D-hexofuranosyl repeating units, hyperbranched D-glucan and D-galactan, respectively. In addition, poly-1 and poly-2 had ca. 30-40 mol % nonreducing D-hexopyranosyl and D-hexofuranosyl terminal units, and the degree of branching was ca. 0.38 for poly-1 and 0.44-0.60 for poly-2. The respective viscosities of poly-1 and poly-2 in aqueous NaNO3(0.2 mol(.)L(-1)) solution were very low with the intrinsic viscosity values of 0.023-0.042 dL(.)g(-1). The steady shear flow of poly-1 in aqueous solution exhibited a Newtonian behavior with steady shear viscosities independent of the shear rate, even at high concentrations. The results indicated that the characteristics of the viscosities were attributed to the spherical structure of the hyperbranched polysaccharide in aqueous solution.