An interaction of Co(II), Ni(II) and Zn(II) with humic acid and O-donor humic-like ligands was studied by ion-exchange and electrospray ionization - mass spectrometry (ESI-MS) method. Interactions were confirmed by differences between values of chromatogram peak areas for monocomponent (ligand) and binary systems (ligand with metal ion) by newly developed ESI-MS 5 μL Loop injection technique. Schubert’s ion exchange method was used for determination of stability constants of formed complexes at pH 4.0 and I = 0.01. Comparing the values of log Kmn for complexes formed by dipositive metal ions with humic, benzoic and salicylic acids, it can be concluded that the interaction strengths of transitional series metals follow the Irving-Williams order: Co(II) < Ni(II) < Cu(II) > Zn(II). The obtained values of log Kmn indicate that Pb(II) ions effect the strongest interactions with all investigated ligands. Complexation of humic acids macromolecules possessing O-donor binding sites as major binding sites can be predicted and modelled following the same order of interaction strength like Irving-Williams order. [Projekat Ministarstva nauke Republike Srbije, br. III41018]