Abstract The phosphomannan-protein complex of Citeromyces matritensis IFO 0651 strain was investigated for its chemical structure by a sequential degradation procedure, partial acid degradation followed by acetolysis under mild conditions. Upon treatment with 10 m m HCl at 100 °C for 1 h, this complex released mannotriose and mannotetraose consisting solely of 1,2-linked β- d-mannopyranosyl residues, ca. 20% on weight basis of the parent complex. The acid-degraded complex was then subjected to acetolysis using an acetolysis medium of low sulfuric acid concentration, a 100:100:1 ( v/v) mixture of acetic anhydride, acetic acid, and sulfuric acid at 40 °C for 36 h. A phosphate-containing mannooligosaccharide fraction eluted in the void-volume region of a Bio-Gel P-2 column was found to consist of Man pβ1→2Man pβ1→ 2Man pα1→2Man to which 1 mol of phosphate group was attached, while a manno-oligosaccharide fraction eluted in the diffusable region was a mixture of Man pβ1→2Man pβ1→2Man pβ1→2Man pα1→2Man, Man pβ1 →2Man pβ1→2Man pα1→2Man, Man pβ1→2Man pα1→2Man, Man pα1→2Man, and mannose in the molar ratio of 0.08:0.33:0.19:0.32:1.00. Therefore, the structural analysis of the polysaccharide moiety of a β-1,2 linkagecontaining phosphomannan-protein complex of fungal origin can be achieved by means of a sequential degradation procedure, partial acid degradation followed by acetolysis under mild conditions.