In this work, the friction theory (FT) and free volume theory (FVT) were combined with the electrolyte perturbed-chain statistical association fluid theory (ePC-SAFT) in order to model the viscosity of pure ionic liquids (ILs) and IL/CO2 mixtures in a wide temperature and pressure (up to 3000 bar) range and with viscosities up to 4000 mPa·s. The ePC-SAFT pure-component parameters for the considered imidazolium-based ILs were adopted from our previous work. These parameters were used to calculate the density and residual pressure of the pure ILs. The density and pressure were then used as inputs for pure-IL viscosity modeling using FVT or FT, respectively. The viscosity-model parameters of FT and FVT were obtained by fitting to experimental viscosity data of imidazolium-based ILs and linearized with the molecular weight of the IL-cation. As a result, the FT viscosity model can more accurately describe the experimental viscosity data of pure ILs than the FVT model, at the cost of an increased number of parameters used in the FT viscosity model. Finally, FT and FVT were applied to model the viscosities of IL/CO2 mixtures in good agreement to experimental data by adjusting one binary viscosity-model parameter between the IL-anion and CO2. The application of FT required fitting the viscosity model parameters of pure ILs to experimental viscosity data of pure ILs and of IL/CO2 mixtures. In contrast, the FVT viscosity model parameters were adjusted to the experimental viscosity data of pure ILs only.