The effect of acetic acid and formic acid on acetone-butanol-ethanol (ABE) production by solventogenic Clostridia was investigated. The ABE concentration in Clostridium acetobutylicum was found to have increased slightly on addition of 3.7 ~ 9.7 g/L acetic acid, but was found to have drastically reduced in the presence of 11.7 g/L acetic acid. However, the solvent production of C. beijerinckii was not affected by addition of acetic acid in the range of 3.7 ~ 11.7 g/L. C. acetobutylicum was more vulnerable to formic acid than C. beijerinckii. In C. acetobutylicum, the total ABE production decreased to 77% on addition of 0.4 g/L formic acid and 25% with 1.0 g/L formic acid. The total ABE production by C. acetobutylicum was also noted to have decreased from 15.1 to 8.6 g/L when 8.7 g/L acetic acid and 0.4 g/L formic acid co-existed. The solvent production by C. beijerinckii was not affected at all under the tested concentration range of formic acid (0.0 ~ 1.0 g/L) and co-presence of acetic acid and formic acid. Therefore, C. beijerinckii is more favorable than C. acetobutylicum when the ABE is produced using lignocellulosic hydrolysate containing acetic and formic acid.