A new voltammetric method is presented for the measurement of humic substances (HS) in natural waters. The method is based on catalytic cathodic stripping voltammetry (CSV) and makes use of adsorptive properties of iron–HS complexes on the mercury drop electrode at natural pH. A fulvic acid standard (IHSS) was used to confirm the voltammetric response (peak potential and sensitivity) for the HS for natural water samples. Optimized conditions included the linear-sweep mode, deposition at −0.1V, pH buffered at 8 and a scan rate of 50mVs −1 . At a deposition time of 240s in the presence of 10nM iron and 30mM bromate, the detection limit was 5μgL −1 HS in seawater, which could be lowered further by an increase in the bromate concentration, or in the adsorption time. The method was used to determine HS in the Irish Sea which were found to occur at levels between 60 and 600μgL −1 . The new method is sufficiently sensitive to detect the low HS content in oceanic samples and has implications to the study of iron speciation.