Reaction with periodic acid-Schiff reagent (PAS), a modified histochemical staining procedure, was used as an index of carbohydrate content of natural waters. Material derived from the standard method for extraction of soluble humic and fulvic acids (HFA) from natural waters, namely adsorption onto and subsequent elution from DEAE cellulose and XAD resin, showed 5-31% of its C content to be carbohydrate. On passage of pure polysaccharide through a DEAE column, 88% of the loading was retained and 50% was subsequently eluted into what would be classified as the HA fraction. Clearly DEAE extraction cannot be regarded as specific for HFA. Similarly, samples were analysed from 62 lentic (non-flowing) waters of varied chemistry and trophic status in NW England. 22%±5% (mean±S.D.) of dissolved organic C (DOC) was PAS+. The proportion of the DOC that reacted positively to the PAS test exceeded 30% of total DOC in sites where DOC concentration was low (<7 mg/1). There was no correlation in the concentrations of DOC and PAS+C suggesting the components of the former have independent sources. Aluminium and Si concentration correlated negatively with PAS+ concentration whilst exhibiting no correlation with DOC. Consequently, prediction of metal speciation in natural waters requires separate consideration of metal interaction with PAS+ and other components of DOC, such as HFA. © 2001 Elsevier Science Ltd.