The possible utilizations of simple manometric devices of the Bial series in investigation of biological decomposition of xenobiotics is described. The study concerns easily decomposing substances and substances potentially degradable. The time course of biological oxygen consumption (BOD) makes possible the determination of final biodegradibility from BOD values in the curve maximum, elimination of the potential accompanying processes (nitrification, respiration of protozoa), and calculation of the kinetic constants of biodecomposition from the 1st order or 2^nd order kinetics. The possibilities were verified in biodecomposition of model substrates and the positive experience is surveyed. The present OECD, ISO and Czech Standard recommendations and standards were considered in the choice of test conditions.