A model for an accumulation and enzyme-activation strategy of a magnetic resonance contrast agent was investigated via the luminescence of Eu(III) analogues. Neutral q = 2 Eu(III) ethyl and acetoxymethyl ester LnaDO3A-based complexes showed increased emission intensity in the presence of serum concentrations of carbonate because of inner-sphere water molecule displacement by the anion. The affinity for carbonate is suppressed by the introduction of negative charge to the complex following enzymatic hydrolysis of the ester groups, resulting in quenching of Eu(Ill) luminescence and changes in spectral form. The conversion of neutral, catoxylic ester-containing complexes into free acid forms by enzymatic hydrolysis using pig liver esterase was demonstrated by luminescence (Eu) and H-1 NMR spectroscopic investigations (Y). These studies demonstrated that the concept of inhibition of anion binding as a result of enzyme activation is feasible.