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The interactions of Co(II), Ni(II) and Zn(II) with humic acid and O-donor humic-like ligands were studied by the ion-exchange and electrospray ionization mass spectrometry (ESI-MS) methods. Interactions were confirmed by differences between the values of chromatogram peak areas for monocomponent (ligand) and binary systems (ligand with metal ion) by a newly developed ESI-MS 5 μL-loop injection technique. The Schubert ion exchange method was used for the determination of the stability constants of the formed complexes at pH 4.0 and I = 0.01 mol dm^-3 . Comparing the values of log K for complexes formed by di-positive metal ions with humic, benzoic and salicylic acids, it could be concluded that the interaction strengths of d-metals followed the Irving-Williams order: Co(II) < Ni(II) < Cu(II) > Zn(II). The obtained values of log K indicated that Pb(II) ions affect the strongest interactions with all the investigated ligands. Complexation of humic acids macromolecules possessing O-donor binding sites as major binding sites could be predicted and modeled following the same order of interaction strength as the Irving-Williams order.