Dextran produced by Leuconostoc mesenteroides NRRL B‐512 was acid‐hydrolyzed and fractionated, yielding a series of fractions from which 24 were selected that ranged in molecular weight from 17,700 to 9.5 million. Light‐scattering and viscosity measurements were made on all fractions, and selected fractions were characterized by endgroup determinations and velocity ultracentrifuge measurements. Branching in these dextran fractions was evidenced by the progressively decreasing slope of the curve of logarithm of intrinsic viscosity plotted against logarithm of molecular weight, attaining a value of 0.11 at M = 107; by the upward curvature of the log‐log plot of sedimentation constant against molecular weight; and by the relatively large values of k′ in the Huggins specific‐viscosity equation for the high‐molecular fractions. Assuming Lansing‐Kraemer molecular‐weight distributions for the dextran fractions and using sedimentation data to furnish β, the measure of breadth of the distributions, physical characterizations were corrected for polydispersity. For molecular weights below 100,000, [η] = 1.0 × 10−3 Mv0.50. In the range 18,000 < M < 400,000 S200 = 0.0251 M00.44. The constant Φ′, of the Flory‐Fox theory, corrected for polydispersity of the fractions, was found to be 55.2 × 1021, which is larger than the value reported for linear polymers. It is concluded that the ratio of root‐mean‐square radius and hydrodynamic radius effective in viscosity differs for branched and linear molecules. The quotient (Φ′)1/2/P′ also appears to be significantly smaller for dextran than the value reported for linear molecules. Increase in Φ′, for dextran above the value previously assumed necessitates recalculation of data of Wales, Marshall, and Weissberg for g, the ratio of mean‐square radii of branched and unbranched dextran molecules.

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