We investigated the conventional characterisation treatability of wastewaters of the "Izmit Industrial and Domestic Wastewater Treatment Plant". Respirometric procedures were carried out for the experimental assessment of organic contents of wastewater. The mean value of total COD was 1201 mg.l(-1) and varied within a wide range of 580 to 1822 mg.l(-1) for the 14 different influents. The soluble COD concentrations were evaluated to vary in the range of 168 to 1601 mg.l(-1), with a mean value of 615 mg.l(-1). The readily biogradable organic substace, S-s is between 0.03 to 0.24% of the influent total COD. The BOD5/total COD ratio in the influent was calculated as 0.20 to be a mean value. The evaluation of this result, wastewater was not suitable for biological treatment. but, the results of COD frationation studies indicate that the organic content of wastewater is mostly diodegradable (84-92%). Therefore, conventional wastewater characterization does not enable for a reliable design of biological treatment processes.