Electrospray-ionisation-mass spectrometry (ESIMS) was used in conjunction with chemical derivatisation and degradation procedures to analyse the size heterogeneity and branching structure of laminarin from the brown alga, Laminaria digitata. Laminarin is a beta-(1-->3)-linked D-glucan with occasional beta-(1-->6)-linked branches. Electrospray-ionisation-mass spectrometry of permethylated laminarin distinguished two homologous series of molecules, a minor G-series containing 22-28 glucosyl residues, and a more abundant M-series containing 20-30 glucosyl residues linked to a mannitol residue. The relative abundance of all these molecular species could be determined simultaneously from a single mass spectrum, with a mean mass error of 0.6 atomic mass units and a mean mass accuracy of 0.011%. Both series had a mean degree of polymerisation of 25 glucosyl residues, and an approximately 3:1 molar ratio of M-series to G-series molecules was maintained across the range of molecular sizes. Treatment of laminarin with periodate, followed by reduction with borohydride, degraded terminal glucosyl residues on both the main chain and the branches, and allowed the detection of isomers differing solely in their degree of branching. M-series molecules were thus shown to contain 0, 1, 2, 3 or 4 branches, with an average of 1.3 branches per molecule; branched G-series molecules were also detected. Subsequent treatment with acid (Smith degradation) showed that 75% of the branches were single glucosyl residues. This study thus shows how the speed, resolution and mass accuracy of electrospray-ionisation-mass spectrometry can be used in the detailed structural analysis of a polydisperse polysaccharide.