We performed the synergistic extraction of lactic acid with tri-n-butylphosphate (TBP) and alkylamines to find an optimum synergistic extraction system. We examined 12 alkylamines as co-extractants and found that extractability with alkylamines increases in order from tertiary > secondary > primary. Extractability increased with increasing alkyl chain length in trialkylamine; tri-n-octylamine (TOA) enabled the best extractability, however, further increases in chain length resulted in a decrease in extractability. The diluent effect was small due to a high concentration of TBP. We found that the synergistic extractions of lactic and succinic acids with TOA and TBP are exothermic processes. (C) 2003 Elsevier Science B.V. All rights reserved.