The present invention provides improved methods for producing a solution containing high molecular weight isolated Streptococcus pneumoniae capsular polysaccharides having phosphodiester linkages between saccharide repeat units. In certain methods, CO.sub.2 is supplied to a fermentation culture of Streptococcus pneumoniae bacterial cells that produce capsular polysaccharide serotypes containing phosphodiester linkages between saccharide repeat units. Exemplary Streptococcus pneumoniae serotypes containing a phosphodiester linkage between saccharide repeat units include serotypes 6A, 6B, 19A, and 19F. Supplying CO.sub.2 to the fermentation culture includes adding bicarbonate ions to the fermentation culture, adding carbonate ions to the fermentation culture, adding mixtures of bicarbonate and carbonate ions to the fermentation culture, and overlaying the fermentation culture with CO.sub.2.