Ion-specific electrodes were used to measure the heavy metal (HM) binding capacity of river waters near Ottawa. Binding capacity was measured in unfiltered water and in water passed through filters retaining particles (0.45 μm) and macromolecules of molecular weight (MW) 45,000, 16,000 and 1,400. In the most studied water samples, almost all the Hg2+-binding ability passed through the smallest filter. Filters of different pore sizes retained substantial fractions of the binding ability towards other HM ions. Binding strengths and conditional binding constants were calculated for each HM ion and low MW Ottawa River water components.Binding in Ottawa River water was not due to HCO3− or CO32− ions; in the Rideau Canal, and probably in other bodies of water, such ions caused a substantial amount of binding. After complete ashing of Ottawa River water and reconstitution with deionized water almost all the HM binding ability was lost; thus, an organic compound(s) is responsible for binding.The binding pattern towards different HM ions of fulvic acid isolated from soil was different from that of unfiltered or filtered Ottawa River water. Fulvic acid is not the sole binding component of this water. These experiments suggest a way of assessing the importance of fulvic acid and other humic substances in HM binding by natural waters.