Fully methylated water soluble seeds polysaccharide with sulphuric acid from Cassia glauca Lam. seeds and precipitated with ethanol then hydrolysed by sulphuric acid. After analysis with Gas Liquid Chromatography, Column and Paper chromatographic analysis of hydrolysate was to be composed of D-galactose and D-mannose in the molar ratio of 1:4. Studies of methylation of polysaccharide by Haworth’s, Hakomari’s and Purdie’s method and its IR-spectroscopy and ¹H and ¹³NMR Spectroscopy indicated that the polysaccharide was a galactomannan with a chain of D-mannopyranose and D-galactopyranose residue linked β-type with (1→4), which carried alternatively α-type with (1→6)-D-galactopyranose residue. Methylated galactomannan on acid hydrolysis (H₂SO₄) gave important medicinal chemicals like methyl sugars as: 2, 3, 4, 6-tetra-O-methyl-D-galactose; 2, 3, 6-tri-O-methyl-D-mannose and 2, 3-di-O-methyl-D-mannose in 1:3:1 molar ratio. On the basis of above finding methylation results a tentative water soluble seeds polysaccharide structure has been proposed for Cassia glauca Lam. Plant.