Various agitation schemes used in digester operation have led to the possibility that carbon dioxide levels may have an effect on digester efficiency. Three digesters were set up in the laboratory, each having a different level of carbon dioxide in the sludge solution. Based on previous work, the digestion processes were interpreted by means of oxidation-reduction potential rather than gas production. An interpretation of the data, indicating that differences found between gas agitation and mechanical mixing are due to physical and not chemical variation, is presented.