Because of its strong Coulomb attraction by the positive Gd(III) aqua ion and its strong repulsion by the negative gadolinium diethylenetriaminepentaacetate (Gd‐DTPA) complex, the trifluoroacetate anion CF3COO− has very different relative spatial dynamics with respect to these two species. Therefore, the Gd(III) aqua ion and Gd‐DTPA complex cause very different longitudinal relaxivities of the trifluoroacetate 19F nuclei. In the presence of competitor Lu(III) ions, the relaxivity difference makes it possible to assay the concentrations of Gd(III) aqua ion and Gd‐DTPA complex from the measured paramagnetic relaxation rate enhancement of the trifluoroacetate 19F nuclei. This experimental method allows one to characterize the present transmetallation equilibrium and it can be easily generalized to other systems.