We introduce a mechanism for ion sensing by MRI in which analytes compete with paramagnetic ions for binding to polydentate chelating agents. Displacement of the paramagnetic ions results in alteration of solvent interaction parameters and consequent changes in relaxivity and MRI contrast, The MRI changes can be tuned by the choice of chelator. As an example, we show that calcium-dependent displacement of Mn2+ ions bound to EGTA and BAPTA results in a T-1-weighted MRI signal increase, whereas displacement from calmodulin results in a signal decrease. The changes are ion selective and can be explained using relaxivity theory. The ratio of T-2 to T-1 relaxivity is also calcium-dependent, indicating the feasibility of "ratiometric" analyte detection, independent of the probe concentration. Measurement of paramagnetic ion displacement effects could be used to determine analyte ion concentrations with spatial resolution in opaque specimens.