Many methods exist for characterising ‘humic’ substances (also known as refractory organic matter, ROM), yet quantification methods in waters are thin on the ground. Since, in common with existing techniques for other categories of natural organic matter, quantification methods do not provide absolute values but values that are always ex- pressed in standard equivalents, the choice of the calibration standard is particularly important, and yet this is often neglected. We recommended analysis of IHSS standard and reference substances close to the origin of the water for calibration purposes when applying our quantification method based on measuring the peaks obtained by adsorp- tive stripping voltammetry of the complex formed by ROM in the presence of trace amounts of Mo(VI). This calibration strategy relies on the assumption that substances with a similar origin will always give close responses. In order to test it, we have applied our method to a series of 18 well-characterised humic acids isolated from a semiarid freshwater wetland (Tablas de Daimiel, Spain). The analytical response obtained is in complete agreement with that obtained from IHSS standard and reference substances of similar origin (i.e., soil and peat), thus confirming the consistency of the approach