Cyclometalated Ru(II) derivatives of 2-phenylpyridine (Hphpy) [Ru(phpy)(bpy)2]Cl (1a) and [Ru(phpy)(phen)2]Cl (1b) (bpy is 2,2'-bipyridine, phen is 1,10-phenanthroline) behave as noncompetitive inhibitors of glucose oxidase from Aspergillus niger in the enzyme-catalyzed oxidation of D-glucose by O2 into the corresponding lactone at pH 5.0 and 25 °C. The enzymatic activity has been measured by monitoring the O2 consumption. The inhibition constants K i are 0.036 and 0.017 M for 1a and 1b, respectively, indicating that 1b inhibits the enzymatic activity more efficiently than 1a. The well-known coordination compound [Ru(bpy)3]Cl2 (2) behaves, in contrast, as a competitive inhibitor, with K i = 0.018 M under the same conditions. The monophasic consumption of O2 in the case of 1a, 1b, and 2 is replaced by a distinct two-phase kinetics in the presence of the cyclometalated Ru(III) compound [Ru(phpy)(bpy)2]Cl2 (3), which was obtained from 1a in the presence of a large excess of H2O2 and the iron TAML activator. Interestingly, the rates of the first and the second phases are influenced by 3 in a different way. The rate of the first phase is noticeably higher in the presence of Ru(III), although the dependence is nonmonotonic and maximal acceleration is observed at the lowest loadings of 3. The rate of the second phase decreases monotonically on increasing the concentration of the ruthenium complex in solution. The nonmonotonic action of 3 was confirmed by using the doubly cyclometalated Ru(III) derivative [Ru(phpy)2(bpy)]Cl. The diverse rate variations induced by 3 accounted for acceleration by Ru(III) of the O2 reduction by the reduced form of glucose oxidase during the first phase, which ceases after the enzymatic reduction of Ru(III) to the Ru(II) species, the latter behaving similarly to 1a as the inhibitor of the enzyme.