Uptake of organic acids by Clostridium acetobutylicum B18 was studied at controlled pH and under reduced butanol inhibition conditions. A pervaporative membrane module was placed in the fermentor to remove butanol from the fermentation broth. Uptake of added butyric acid followed zero order kinetics at pH 4.75 and first order kinetics at pH 5.75. At pH 5.25 the kinetic order shifted from zero to first order as the butyric acid was taken up. At the point of order shift undissociated butyric acid(UBA) concentration was approximately 0.5g/L. Unlike butyric acid, uptake of acetic acid followed first order kinetics regardless of pH. The difference in acid uptake kinetics could be explained by the combined effect of acid diffusion across the cell membrane and intracellular enzymatic reaction. The acid concentration for kinetic order shift seemed to be dependent upon pH and the kind of the acid used. Glucose was consumed simultaneously with added acids. Both butyric and acetic acids were taken up simultaneously but the rate was faster for butyric acid. Added butyric acid was completely assimilated whereas acetic acid uptake was incomplete.