Paramagnetic and magnetic nanoparticles may be used as contrast agents in porous media to improve NMR measurements. In rocks, magnetic and paramagnetic cations in pore fluid may interact with nanoparticles and affect the magnetic resonance properties of the nanoparticles, which in turn makes interpretation of NMR measurements difficult if this effect is not accounted for. In this work, two groups of zirconia nanoparticles: one with positive surface charge, the other with negative surface charge; and a group of silica nanoparticles coated with poly(ethylene glycol) (PEG) and negatively charged were mixed with Fe(III) solution for interaction and/or adsorption to occur. T1 values of fluid with different concentrations of Fe(III) ranging from 9 mg/L to 50 mg/L were obtained, and we confirmed a linear relationship between Fe concentration and T1 values at fixed pH conditions. The T1 values of zirconia and silica nanoparticle dispersions before and after mixing with various Fe(III) solutions were measured and compared. Adsorption of iron onto zirconia nanoparticles was confirmed based on measurements of aqueous Fe remaining in supernatants. The response of silica nanoparticles mixed with Fe solutions was also consistent with adsorption of Fe ions on the silica nanoparticle surfaces. Adsorbed iron increases zirconia nanoparticles’ surface relaxivity by more than 50 fold, and the relaxation rate of zirconia nanoparticles increased with the amount of adsorbed Fe(III).