Different polysaccharides containing D-mannose residues have been C-6-oxidized by a selective TEMPO-mediated hypohalite oxidation to obtain the corresponding uronans. These have been treated with various recombinant mannuronan C-5-epimerases and the resulting products were analyzed by H-1 NMR spectroscopy. Oxidized konjac mannan could be epimerized to obtain a uronan with a content of about 12% alpha -L-gulopyranuronate (G) residues. On prolonged epimerization, beta -elimination was observed. The oxidized galactomannan locust bean gum could only be scarcely epimerized, probably due to steric effects exerted by its 26% alpha -D-galacturopyranosyl side groups. Oxidized, galactose-depleted guar gum with a a-D-galactosyl content of 11% could be epimerized to a G content of about 15%. With oxidized cellulose as a substrate, mainly beta -elimination was observed. It thus seems that the mannuronan C-5-epimerases employed recognize glucuronate residues and abstract proton-5 but are unable to perform the second epimerization step and instead yield beta -eliminated products.