Tetrazolium reduction and enzyme kinetics were examined to estimate the ETS activity of decomposing reed (Phragmites australis /Cav./ Trin. ex Steud.) rhizome to collect information on the activity of microbial decomposers. Optimal incubation time was determined at 22 °C. For complete enzyme extraction, 4-6 min. of homogenization was necessary. The main substrates of the enzymatic reaction were NADH and NADPH. The reaction was fastest when 2-(-p-iodophenyl)-3-(-p-nitrophenyl)-5-phenyl tetrazolium chloride (INT ; 0.8 mM), NADPH (0.25 mM) and NADH (1.7 mM) were applied simultaneously. The optimal incubation time should be less than 20 minutes. The pH optimum of the enzyme reaction is 8.0-8.4. ETS activity of decomposing reed rhizome can be used to estimate potentially the oxygen consumption of microorganisms involved in decomposition and, indirectly, the rate of decomposition.