Galactose oxidase was used as a catalyst to oxidize selectively the C-6 hydroxyls of terminal galactose to carbonyl groups. The polysaccharides studied included spruce galactoglucomannan, guar galactomannan, larch arabinogalactan, corn fiber arabinoxylan, and tamarind seed xyloglucan, with terminal galactose contents varying from 6% to 40%. A multienzyme system was used, with catalase and horseradish peroxidase to enhance the action of galactose oxidase. An analysis technique was developed for the quantification of the reactive aldehydes with GC-MS, utilizing NaBD4 reduction and acidic methanolysis. The best oxidation degrees of terminal galactosyls were obtained with xyloglucan (85% of galactose) and spruce galactoglucomannan (65% of galactose). The highest oxidation degree based on total carbohydrates was achieved with guar gum (28%), which had the highest galactose content. The oxidation resulted in changes in the physicochemical properties of the polysaccharide solutions, and the changes observed varied between the polysaccharides. The clearest change was in tamarind xyloglucan, which formed a gel after the oxidation. After the oxidation, larger particles were present in the solution of spruce galactoglucomannan, but changes in its rheological properties were not observed.