in this work, an experimental and theoretical investigation of selectivity was performed for simultaneous absorption of H2S and CO2 from gaseous streams into an aqueous monoethanol amine (MEA) solution in a packed bed. The operating parameters playing a vital role and have been considered here, are amine concentration and pressure, while the structural and hydrodynamic parameters such as packing size and L/G ratio were found to be important as well. The qualitative effects of these parameters on absorption performance were analyzed via evaluating of absolute and relative removal efficiency. in the proposed mathematical model for theoretical investigation of the absorption performance, the two-film theory of mass transfer was used. In order to check the validity of the proposed model, in experimental section, a pilot-scale plant was constructed and variety of experimental data were obtained in a wide range of operating conditions. In this plant, construction of packed beds in several sections of the column, gives the possibility of changing the structural and hydrodynamic parameters in a flexible way. Also the proposed mathematical model is able to consider the effects of vital parameters on the selectivity of absorption. The predicted results by the proposed mathematical model were compared to the experimental data. This comparison shows a good agreement between these data and so verifies the validity of the proposed model. The results reveal that the effects of pressure, amine concentration, packing size and L/G ratio on the selectivity performance of H2S absorption into an MEA solution are significant. Results show that, the relative efficiency could be increased near 20% when L/G is manipulated properly. As a side effect it reduces the absolute efficiency. Using the packing size I in. instead of 0.5 in., relative efficiency could be increased more than 5% while absolute efficiency is reduced. Then the experimental results testified the benefit of any operating procedure appropriately involving these parameters for selective H2S absorption. In general, since CO2 is a weak competitor against H2S in being absorbed by MEA solution, under difficult conditions for absorption process, the weakness Of CO2 is more pronounced. Therefore, the selectivity of absorption is increased, while the absolute efficiency is decreased. In particular, increasing the gas flowrate and decreasing the L/G ratio, increases the selectivity. Also when higher packing size is used, selectivity is increased. In contrast, when large amount of amine are contacted with a special quantity of acid gas in the unit volume of packed bed (generally lower packing size and higher L/G ratio), due to the strength of MEA, the amine solution absorbs nearly all gaseous acidic compounds and reduces the selectivity. (C) 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.