The overexpression of CoA transferase (ctfAB), which catalyzes the reaction: acetate/butyrate + acetoacetyl-CoA -> acetyl/butyryl-CoA + acetoacetate, was studied for its effects on acid reassimilation and butanol biosynthesis in Clostridium tyrobutyricum (Delta ack, adhE2). The plasmid pMTL007 was used to co-express adhE2 and ctfAB from Clostridium acetobutylicum ATCC 824. In addition, the sol operon containing ctfAB, adc (acetoacetate decarboxylase), and ald (aldehyde dehydrogenase) was also cloned from Clostridium beijerinckii NCIMB 8052 and expressed in C. tyrobutyricum (Delta ack, adhE2). Mutants expressing these genes were evaluated for their ability to produce butanol from glucose in batch fermentations at pH 5.0 and 6.0. Compared to C. tyrobutyricum (Delta ack, adhE2) without expressing ctfAB, all mutants with ctfAB overexpression produced more butanol, with butanol yield increased to 0.22 -aEuro parts per thousand 0.26 g/g (vs. 0.10 -aEuro parts per thousand 0.13 g/g) and productivity to 0.35 g/l h (vs. 0.13 g/l h) because of the reduced acetate and butyrate production. The expression of ctfAB also resulted in acetone production from acetoacetate through a non-enzymatic decarboxylation.