Heterogeneous liquid phase selective catalytic oxidation of lactose and/of lactobionic acid towards 1-carboxylactulose (2-keto-lactobionic acid) was investigated. Bismuth-modified platinum catalyst with air as the terminal oxidant was the catalyst of choice to carry out this conversion. The oxidation of lactobionic acid was performed with a selectivity >95% towards 1-carboxylactulose but the conversion did not exceed 50% due to poisoning of the catalyst. When lactose was used as substrate the conversion of lactobionic acid formed in situ was extended to 83% and the catalyst selectivity towards 1-carboxylactulose was maintained. The lactose was completely oxidized to lactobionic acid.