The electrocatalytic reduction of nitrite at an electrode modified with the redox polymer [Os(bipy)₂(PVP)₁₀Cl]Cl, where bipy = 2,2′–bipyridyl, PVP = poly(4-vinylpyridine), is described. A possible mechanism for the mediated reaction is proposed. The kinetics of the mediated reaction are examined and compared to a theoretical rate equation describing the proposed mechanism. The reaction most likely proceeds through the species, NO⁺. Kinetic analysis reveals that the kinetic situation Lk best describes the electrocatalytic reaction, with mediation occurring throughout the polymer film and electrode currents being controlled by the rate constant for the cross-exchange reaction. An equation relating limiting currents to known and easily measurable parameters is also presented which may be used for the internal calibration of sensors constructed from these electrodes.