Stability constants of zinc-humic acid complexes separated from soils,compost,peat and weathered coals were determined by ion exchange equilibrium method.Humic substances studied varied markedly in average molecular weight,content of carboxyl and phenolic hydroxyl group,degree of dissociation of carboxyl group and aromaticity.The results showed that the stability constants of Zn-humic acid complexes varied with the different origins of the humic substances,with the highest logK of 7.85 for a weathered coal and the lowest value of 3.52 for the albic horizon of a albic brown forest soil.The log K was correlated significantly with the content of undissociated (Ar weakly dissociated) carboxyl group,but not with the content of total carbonyl group pr phenolic hydroxyl group,average molecular weight,or aromaticity of Mimic substances.