The effect of pH, ionic strength, and temperature on the kinetics of oxidation reactions between residual lignin and polyoxometalate (POM) anions ([AlMnIII(OH2)W11O39](6-) and [SiMnIII(OH2)W11O39](5-)) in a pulp suspension of unbleached softwood kraft pulp at varying conditions was investigated. According to the results, cation concentration and pH have a remarkable effect on reactivity of the POMs. At cation concentrations <= 0.10 mol L-1, no reaction was observed during the first 5000 s; at 0.30 mol L-1, the reactions were significantly more rapid, and the rate correlated strongly with increasing pH for both POMs. The activation energies (E-a) of the oxidation reactions at 0.30 mo L-1 cation concentration varied between similar to 27 and 68 kJ mol(-1) for [AlMnIII(OH2)W11O39](6-), depending on pH and reaction time; the respective values for [SiMnIII(OH2)W11O39](5-) were similar to 17 to 82 kJ mol(-1). The reasons for the observed decrease in E-a along with increasing pH and cation concentration are discussed.