Lanthanide complexes of DOTA monoesters bearing nitrobenzyl and nitroimidazole groups are shown to be converted to the corresponding DOTA complexes under chemical and enzymatic conditions, giving rise to favorable changes in the luminescence properties of the europium and terbium complexes and relaxometric properties of the gadolinium complexes. The nitroimidazole complexes are converted more rapidly than their nitrobenzyl and benzyl analogues. We propose that activation of these complexes may occur by ester cleavage rather than nitro reduction and fragmentation since complexes bearing a simple benzyl group may also be cleaved under the same conditions, albeit more slowly.