The conformational dynamics of NADH oxidase from Thermus thermophiluswas modulated by the Hofmeister series of anions (H2PO4 – ,SO4 2– ,CH3COO – ,Cl – ,Br – ,I – , ClO4 – ,SCN – ) in the concentration range 0–3M. Both cha-otropic and kosmotropic anions,at high concentration, inhibit the enzyme by different mechanisms. Chaotropic anions increase the apparent Michaelis constant and decre-ase the activation barrier of the reaction. Kosmotropic ani-ons have the opposite effect.