Technical Abstract: Alternan is an ultra-high molar mass polysaccharide composed of two different types of glycosidic linkages and which also possesses long-chain branching. It has great potential in a number of areas, including as a possible domestic replacement for gum Arabic. The high molar mass and large size of alternan, however, complicate its characterization by traditional techniques such as size-exclusion chromatography, wherein the polysaccharide undergoes on-column, flow-induced degradation even using the largest particle size columns available at very low flow rates. To overcome the problems accompanying SEC characterization of alternan, we employed packed-column hydrodynamic chromatography with multi-angle static light scattering and differential refractometry detection. The Mw and RG,z values obtained by HDC/MALS were closer to the values obtained by off-line MALS analysis than were the Mw and RG,z determined by SEC/MALS. Moreover, analysis by HDC was completed in a fraction of the time needed for SEC analysis. Evidence for long-chain branching in alternan was seen in the intrinsic viscosity of the polysaccharide, which is over 300 times lower than would be that of an equivalent-M pullulan, and in the viscometric radius of alternan, which was approximately one seventh that of the equivalent-M pullulan.