The kinetics of hydrogen peroxide decomposition by heat-treated Co-tetramethoxylphenyl-porphyrin(Co-TMPP)/ carbon black and Fe-tetraphenyl-porphyrin(Fe-TPP)/carbon black has been studied by a gasometric method in sulphuric acid. In the former case, the reaction order is 1.26 +/- 0.11, indicating that a combined homogeneous and heterogeneous mechanism is involved. In the latter case, the reaction order is 1.02 +/- 0.11, indicating that a heterogeneous mechanism is involved. The nature of the dissolved species from the heat-treated sample has been studied using UV-visible spectroscopy.