A comparative study has been carried out for two sets of humic acids isolated from lignites and soils. H-binding data were analyzed using the NICA-Donnan model, for three Greek lignite humic acids (HA) Plus IHSS Leonardite reference HA, and five Greek soil HAs plus a commercial peat HA. C-13-CP-MAS NMR and H-binding data provide quantitative estimates for functional groups, showing that lignite HAs of diverse origin have strikingly homogeneous properties, while the H-binding structural units of soil HAs are characterized by a large degree of variability. Consistent differences between soil HA vs. lignite HA are revealed at the level of functional groups' concentrations. In the pH range 4 to 10, soil HA showed a charge variation < 3 [equiv kg(-1) ] while lignite HAs showed a higher charge variation > 3.5 [equiv kg(-1)]. (c) 2008 Elsevier Inc. All rights reserved.