The rate constants for the oxidation of pentacyano(N-methylpyrazinium)ferrate(II) by hydrogen peroxide and the substitution by dimethyl sulfoxide under saturation conditions are the same (k = (3.8 +/- 0.1) x 10(-4) s-1 at pH 7.1 (MOPS buffer), 25-degrees-C, and I = 1.0 M by use of LiClO4). The peroxide reaction is not catalyzed by Mo(VI) or W(VI). The kinetics of the oxidation of hexacyanoferrate(II) by hydrogen peroxide were monitored under conditions where catalysis by pentacyanoaquoferrate(II) was neglible. The kinetic traces were approximately pseudo-zero-order under typical conditions (2.00 X 10(-3) M Fe(CN)6(4-), 1.00 x 10(-2) M H2O2, 5.0 X 10(3) M Mpyz+, pH 4.0 (acetate buffer), and 25-degrees-C). The slopes (a) were not very reproducible, (b) varied with the initial concentration and source of the hexacyanoferrate(II), (c) were linearly dependent on the concentration of added Mo(VI) or W(VI), (d) were too large for the values expected for an outer-sphere direct reaction between the reductant and hydrogen peroxide, and (e) were substantially decreased by the presence of metal ion chelators. It was found through studies with added iron(III) or iron(II) that the reaction rates are catalyzed and largely driven by trace metal ion contaminants in the absence of chelating agents. Attempts to measure the true rate of oxidation of hexacyanoferrate(II) by hydrogen peroxide were unsuccessful, but it was shown that the upper limit established is more in line with that expected for the outer-sphere pathway. Some preliminary studies of the very slow oxidation of hexacyanoruthenate(II) by hydrogen peroxide are presented.