Hydrogen sulfide is part of a well-known environmental problem afflicting pulp mills exploiting the Kraft mill sulfate-pulp process. Among the total reduced sulfurs (TRS) quartet (H2S, CH3SH, CH3SCH3, CH3S2CH3) hydrogen sulfide is the most abundant component in the effluents. Utilization of Fe(111) chelate complex of trans-1,2-cyclohexanediaminetetraacetic acid (CDTA) for the oxidation of hydrogen sulfide is beneficial from the standpoint of iron-sequestration and protection against precipitation in the alkaline environments characteristic of the Kraft mill streams. The physical solubility of H2S in CDTA-Fe(III) complexes, which is key for designing scrubbing-absorption, cannot be measured directly because of oxidation of the sulfur-bearing gaseous species with the ferric chelate. Therefore, the physical solubility of hydrogen sulfide in aqueous solutions in which H2S does not react (Fe(II) complexes of CDTA) was studied at temperatures from 293 to 323 K and sub-atmospheric pressures. New experimental results over iron-free CDTA and CDTA-Fe(II) chelate concentrations ranging from 38 to 280 mol m(-3) are reported. The Henry's law constants and thermodynamic solution properties of H2S in these solutions were correlated as a function of temperature and chelate concentration. (C) 2004 Elsevier B.V. All rights reserved.