Interfacial tension (IFT) between reservoir fluids is an important property in enhanced oil recovery (EOR) and carbon geological storage (CGS). Quantitative knowledge of IFT is needed to support and assist the interpretation of multiphase flow and wetting behaviour in porous media and to facilitate numerical reservoir simulation. Iododecane and iodide-containing brines are common contrast agents in visualisation of multiphase flow in porous media by X-ray CT imaging. The effect of the introduced contrast agents on the IFT was studied in this work by means of pendant-drop experiments and modelling with the density-gradient theory. We report experimental IFTs between N2, 7 mass% KI (aq), and decane-iododecane mixtures with various iododecane mass fractions at temperatures from 298 K to 353 K and pressures from 1 MPa to 30 MPa. The IFTs between N2 and the liquid phases decrease with the increase of either pressure or temperature and increase with the increasing KI molality or iododecane mass fraction. The IFTs between H2O and decane-iododecane mixtures decrease with temperature or iododecane mass fraction and increase slightly with increasing pressure. The IFT data were modelled by means of the density-gradient theory coupled with the volume-translated Peng-Robinson equation of state. Empirical equations were also developed to correlate all of the measured data. A workflow was proposed for estimating the IFTs between gas, brine and the doped hydrocarbon systems based on the experimental and modelling work.