Advantages of the described method of COD (cr) determination using microwave heating (550 W, 2 min of exposure) consist in a considerable time saving in the analysis and reduced consumption of the mercury (II) salt because the interference of chlorides is substantially lower under conditions used. Comparison with the approved method is, however, inevitable for the real types of water samples, above all for samples from biological sewage-treatment plants. The aim of the method is not to replace the standard procedure, but the application in research water-supply laboratories (e.g., in the study of biological degradation of organic substances) or for instant tests of recipient pollution or technological water-treatment process.