The marker enzyme of the peroxisome—a phylogenetically old yet only recently discovered cell organelle—is catalase, a hemoprotein which decomposes hydrogen peroxide catalatically as well as peroxidatically. In the peroxisomes, catalase is associated with H2O2-producing oxidases and other enzymes. Also in parenchymal cells such as liver and kidney cells part of the reduction of oxygen occurs via formation of H2O2. A central role in peroxisomal H2O2-metabolism is played by the active intermediate, catalase-Fe3+-H2O2, (Compound I), which is distinguished from free catalase by specific absorption bands. Organ photometry on intact hemoglobin-free perfused rat liver in order to measure Compound I selectively provides a direct insight into the dynamics of the H2O2 metabolism which takes place in the range of nanomolar concentrations. Endogenously, 1g of liver forms approximately 50 nmol of H2O2 per min. The turnover number, which in the steady state is < 10 min−1 in the cell as compared to > 108 min−1 for the isolated enzyme with an excess of substrate, can be increased to approximately 102 min−1 by intracellular stimulation of the H2O2 production (e.g. by glycolate or urate). The peroxidatic oxidation of hydrogen donors (e.g. methanol and ethanol), favored relative to the catalase pathway at low turnover numbers, is of importance in normal metabolism and in pathological conditions.