The effect of organic matter (pig manure,Sus scrofa) addition on solubility and free Cd(II) and Zn(II) speciation was studied in two mineral soils. The soils were extracted with ultra pure 0.01MKNO3, and the extracts were analyzed for total dissolved Cd and Zn by graphite furnace AAS and ICP, respectively, and for labile Cd and Zn by differential pulse anodic stripping voltammetry (DPASV). Based on the assumption that the non-ASV-labile fraction of the total dissolved Cd and Zn was organically bound to fulvic acid (FA), the relative fraction of labile to total dissolved Cd and Zn was used to estimate conditional stability constants (log K) for the formation of Cd-Fulvate (CdFA) and Zn-Fulvate (ZnFA). Species of organically and inorganically associated Cd and Zn, as affected by the addition of pig manure to the two different soil types, were calculated. The addition of organic matter increased the solubility of Cd and Zn in both soils by the formation of organo-metallic complexes. The lability of Zn was, however, reduced substantially, whereas for Cd it was unaffected. The conditional log K values calculated indicate that the stability of organo-metallic complexes with Cd and Zn may be more important than reported previously. This implies that increasing concentrations of dissolved organic acids can increase their solubility, thus leading to the leaching of Cd and Zn into ground water.