Addition of acetate or propionate to uncontrolled-pH batch cultures does not affect the initiation of solventogenesis but does enhance final solvent concentrations compared with those of unchallenged cultures. This observation can be explained in terms of the increased buffering capacity of the medium brought about by the added acids, resulting in protection against premature growth inhibition due to low culture pH values at the end of the fermentation. The uptake of propionic acid from the medium does not proceed solely via the coenzyme A-transferase pathway, since less acetone than propanol is formed. Therefore, at least 50% of the propionic acid is taken up through the reversed kinase-phosphotransbutyrylase reaction pathway.