New experimental results for the solubility of hydrogen sulfide in aqueous solutions of piperazine are presented for piperazine molalities up to 4 mol/kg of water, temperatures between 313 and 393 K, and pressures up to about 9 MPa. Furthermore, the solubility of hydrogen sulfide was measured in about 2 m MDEA + 2 m piperazine aqueous solutions at 353 K up to about 6 MPa. Considering the new experimental data, a thermodynamic model for describing phase equilibria for the system H2S-methyldiethanolamine-water has been extended allowing for the presence of piperazine. (C) 2003 Elsevier Science B.V. All rights reserved.