In addition to d-glucose, l-rhamnose, and phosphate, previously shown to be present in the specific polysaccharide of type XVIII pneumococcus (S XVIII), galactose and glycerol are now identified chromatographically. Treatment with alkali yields glycerophosphate and undialyzable polymer containing glucose, galactose, and rhamnose. This polymer precipitates only 15% of the antibodies in Type XVIII antipneumococcal horse serum reactive with intact S XVIII. It also shows increased cross-reactivity with a Type VI antipneumococcal serum which contains antibodies cross-reactive with S II and other polysaccharides with multiple residues of (1 → 3)-linked l-rhamnose. Oxidation of alkali-degraded S XVIII with periodate leaves rhamnose and galactose in resistant linkages and destroys glucose, with production of erythritol and glycerol. The chemical properties and immunologic cross-reactivities of S XVIII and its product of alkaline degradation indicate that a part of the glucose is probably linked (1 → 4)- and (1 → 6)-and that the rhamnose and galactose are bound (1 → 3)-. © 1962, American Chemical Society. All rights reserved.