The degradation rate constants of the tetracations 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin and 5,10,15,20-tetrakis(N,N,N-trimethylanilinium-4-yl)porphyrin as well as their manganese(III) complexes toward the oxidants H2O2, t-BuOOH, and NaOCl were measured at various pH values, I = 0.2 M and 30 degrees C. The free-base and manganese porphyrins showed high structural stabilities toward H.,02 and t-BuOOH in strong acidic solutions and their degradation constants were on the order of 10(-6) min(-1) or no porphyrin degradation was observed at all. As the pH increased, their stability towards these peroxides decreased and in basic solutions, they showed almost no stability toward H2O2, and a moderate stability toward t-BuOOH. When NaOCl was employed as the oxidant, they showed only moderate stabilities in strong basic solutions and almost no stability in slightly basic, neutral or acidic solutions. The degradation rate constants in the presence of NaOCl in strong basic solutions varied from 10(-2) min(-1) to 101 min(-1). Copyright (c) 2004 Society of Porphyrins & Phthalocyanines.