Galactose oxidase is a fungal enzyme which is known to oxidize the C-6 hydroxymethyl of galactose to an aldehyde group. When the products of a galactose oxidase-catalase treatment of raffinose were examined by gel filtration and ion exchange chromatography, we found that, in addition to the expected 6''-aldehydoraffinose, two other components were present. Of these two components, the major one was retained on a column of AG 1-X8 (formate), gave a positive carbazole reaction for uronic acid, and on paper chromatograms had a mobility identical with that of 6''-carboxyraffinose. The infrared spectrum of the compound showed a carbonyl absorbance at 1725 cm-1 and was distinguishable from the spectra of raffinose and 6''-aldehydoraffinose. These data showed that raffinose was partly converted to 6''-carboxyraffinose when treated with galactose oxidase and catalase. The conversion of [3H]raffinose to [3H]6''-carboxyraffinose increased gradually with time of oxidation from 22% at 6 h to 68% at 96 h. Results of other experiments provided evidence that this was an enzymic conversion and depended on the presence of galactose oxidase. The activities responsible for the formation of aldehyde and uronic acid could not be separated by affinity chromatography, gel electrophoresis, or ion exchange chromatography, indicating that the same enzyme is responsible for both activities. Treatment of galactose, melibiose, and stachyose with galactose oxidase and catalase also resulted in the formation of the corresponding uronic acids. These studies indicate that galactose oxidase not only converts the C-6 hydroxymethyl group of galactose to an aldehyde group, but also catalyzes further oxidation to the carboxyl group.