The molecular weight distributions of four polymethylmethacrylate fractions have been studied by column elution chromatography and by sedimentation velocity in an ultracentrifuge.The criterion for the absolute evaluation of the molecular weight distribution curves obtained is based on the superpossibility of the distribution curves of single fractions to give the distribution curve of a mixed fraction.The solvents used for gradient elution were benzene-methanol, while ultracentrifugal sedimentation was observed in acetone-ethanol at the vicinity of B-temperature of the polymer-solvent system.The experimental results indicate that the fractionation curves from elution chromatography gave too narrow a distribution in molecular weights, while the sedimentation velocity method gave very satisfactory results.It is concluded that if the conventional method of calculating the integral distribution curve from the fractionation data is used, the high molecular weight tail of the distribution curve is greatly underestimated.A better procedure of treating the fractionation data with due allowance for the molecular weight distribution within each fraction remains to be devised.When this is done, the present authors expect to get reliable results for the molecular weight distribution of a polymer sample by simple fractionation into a few fractions.