The well-known perturbed-chain-statistical-association-fluid-theory (PC-SAFT) EoS was employed to model CO2 and H2S absorption in some protic ammonium-based ionic liquids, including Methyl-diethanol-ammonium Formate, Methyl-diethanol-ammonium Acetate, Dimethyl-ethanol-ammonium Formate, and Dimethyl-ethanolammonium Acetate. In this way, all the acidic gases and the ILs were considered as associative compounds so that they can establish hydrogen bonding by their own or other molecules. Accordingly, 4C, 2B, and 1A association schemes were assumed for H2S, the ILs, and CO2, respectively. Moreover, to estimate the liquid phase concentrations, a complex formation reaction (CFR) approach was followed. In this concept, acidic gases are supposed to form chemical complexes with the ILs. To show the importance of the reactions, the solubilities were calculated with and without using them, and the achieved results were compared. As the outputs show, considering the reactions, the excellent overall AADs% equal to 1.38 and 0.17 were obtained for H2S and CO2 absorption, respectively. While without them, these values were about 5.05 and 8.57. In the second part of the work, the viscosity of the used ILs was estimated through a new approach that combines the free-volume-theory (FVT) and the PC-SAFT, CPA, and mSRK EoSs. Accordingly, the density of the ILs computed using the EoSs, and then FVT was applied to estimate the dynamic viscosity. Based on the outputs, all the EoSs illustrate good ability to calculate viscosities, precisely, so that all the models present AAD% of about 2.5. This is because of the high ability of the FVT to obtain a precise estimation of viscosity using a rough estimation of the density.