The complexation parameters (complexation capacity, , and conditional stability constant, K̃) for binding of Ni(II) and Co(II) by natural organic matter (NOM) are shown to be dependent on the analytical detection window used for their determination. This observation is a consequence of ligand heterogeneity. Complexation parameters for isolated NOM components (humic substances and biota) are typically determined at low detection windows (weak, labile binding sites detected), whereas whole waters are generally studied at much higher detection windows (stronger, less labile sites detected). Comparison and interpretation of metal ion binding across systems and across metals is only valid if all the data were obtained within the same detection window.