Solubility of the single gases CO2 and H2S in the ionic liquid 1-butyl-3-methylimidazolium tri-fluoromethanesulfonate ([C(4)mim][OTf]) was measured at temperatures from (303.15-343.15) K and pressures up to about 3.2 MPa for CO2 and 1.6 MPa for H2S. A modified isochoric saturation technique was developed in this study to determine the volume of the gas saturated liquid phase in the solubility measurements. The reliability of the experimental setup and measurement method was assessed through detailed comparison of generated data for CO2 + [C(6)mim][Tf2N]binary mixture with the most reliable data reported in the literature. The average of percent absolute relative deviation (AARD%) of pTx and specific volume data (in cm(3). kg(-1)) measured in this work from the corresponding data in the literature is 3.7% and 0.23%, respectively. Comparison of the obtained solubility data shows that H2S is more than 4 times as soluble as CO2 in [C(4)mim][OTf]. The solubility of both gases in [C(4)mim][OTf] is higher than in the lower homologue [C(2)mim][OTf]. Furthermore, [C(4)mim][OTf] shows the highest absorption capacity for H2S and high H2S over CO2 selectivity relative to other [C(4)mim]/[C(2)mim] - based ionic liquids with different anions. The experimental data were correlated by using the extended Henry's law combined with a simplified Pitzer's model. The values of AARD% of correlated molalities from experimental data are 1.1% for CO2 + [C(4)mim][OTf] and H2S + [C(4)mim][OTf] binary mixtures. The Henry's law constants, partial molar volume at infinite dilution and thermodynamic functions for dissolution of CO2 and H2S gases in [C(4)mim] [OTf], as a function of temperature, were calculated from the experimental data. (C) 2017 Elsevier B.V. All rights reserved.