A spectrophotometric method of monosaccharide analysis was assessed with regard to its non-selectivity towards humic substances. Analysis of model solutions showed that it responds positively to both marine and terrestrial humics. The systematic error in monosaccharide analysis (in glucose equivalents) was 0.18-0.20 mg per 1 mg of Aldrich humic acid and 0.11-0.12 mg per 1 mg of humic substances isolated from Gulf of Gdańsk water.