Partition coefficients represent a convenient way to describe metal distribution among the particulate, colloidal and truly dissolved phases in aquatic systems (see footnote 1 for definitions). In particular, the distinction between colloids and true solution is extremely relevant to element fate and bioavailability. Regrettably, measurement of colloidal and truly dissolved metals is usually not performed in routine monitoring programmes since it is costly and time consuming. We show the possibility to estimate truly dissolved metal concentrations from the more easily measured particulate and filterable metal concentrations for a range of freshwater systems and one estuary.