Biofilms envelop all surfaces in aquatic ecosystems. They possess an extremely efficient nutrient entrapment mechanism which is widely believed to be mediated through ion exchange processes. During a field experiment, potassium and bromide were transported along a 105 m reach at different rates. The distance between the two solute pulses increased with increasing distance downstream. And, in a laboratory experiment, changing the ionic composition of waters overlying the biofilm influenced the retention of (phenolic) material by that biofilm. An analogy was drawn with ion chromatography (IC): In IC, different ions show different rates of progress through the column (retention times), and also show increasing separation between peaks with increasing distance from the point of injection (column length). Likewise, the affinity of a given ion for the column can be modified by manipulation of the ionic composition of overlying waters (eluent). The observed similarities between IC columns and the biofilm-coated stream channel may therefore represent a degree of experimental support for the putative involvement of ion exchange in the biofilm nutrient entrapment mechanism.