The carbon dioxide produced during biological oxidation of wastewater has a pronounced influence upon the pH that is attained in the activated sludge process. The quantity produced is proportional to the COD removed, its degree of oxidation and depends also on the oxidation level of the substrate. A discussion of metabolism is given with emphasis on the description of the oxygen consumption. The theory is compared with experimental results. A mass balance for the metabolism of model compounds is presented. This model is based on a thermodynamic consideration of the metabolic process and offers a basis for calculation of R. Q. values. Experimental evidence for the applicability of this model is presented. Experimental results for a sewage are applied in pH calculations.