Our recent achievements concerning the synthesis, characterization and exploration of some biochemically significant features of peroxo niobium (PNb) complexes are highlighted here. Water soluble heteroleptic PNb complexes in a variety of ligand environment were screened for their activity with two types of enzymes viz. calcineurin and acid phosphatase. Employing wheat thylakoid ACP as the model enzyme, it has been demonstrated that peroxoniobium derivatives serve as active inhibitors of phosphatase (IC50 values 2–9 M). Enzyme kinetics data revealed that compounds exert mixed type of inhibition on ACP activity (Kii > Ki). Calcineurin inhibitors represent a valuable tool for elucidating CN dependent cellular processes. In vitro effect of the complexes on CN activity was examined using physiological substrate RII-phosphopeptide. As revealed by enzyme kinetic investigation, hydrogen peroxide as well as the peroxoniobium derivatives, irrespective of the nature of their ligand environment, inactivated calcineurin exclusively via an uncompetitive pathway.