An already established analytical method for the quantification of freshwater refractory organic matter (often called humic substances) has been applied to organic matter from a wastewater treatment plant's effluent and a number of downstream locations impacted by the treatment plant. The method is based on measuring the peak currents obtained by adsorptive stripping voltammetry of the complex formed by freshwater refractory organic matter in the presence of trace amounts of Mo(VI). Organic matter is first concentrated by reverse osmosis, then fractionated according to its polarity by the sequential application of DAX-8 and XAD-4 resins. The results obtained show that the voltammetric method measures the refractory organic matter present in the different sewage-derived fractions and that the response obtained follows the trend expected for the fractionation method used (i.e. more hydrophobic fractions largely give the strongest signals). These results have been compared with those obtained from IHSS substances. The results of this study are of particular significance in that it shows that, when applied to surface waters, the voltammetric method measures any refractory organic matter present, irrespective of its origin.