Formic, acetic, and succinic acids have been selectively separated from their mixture obtained by A. succinogenes fermentation using reactive extraction with tri-n-octylamine (TOA) dissolved in three solvents with different polarities (n-heptane, butyl acetate, and dichloromethane) without and with 1-octanol addition. This technique allows recovering formic and acetic acids from the mixture, the raffinate containing only succinic acid. The extractant concentration and organic phase polarity control the selectivity of acids extraction. Thus, at pH=1, the selectivity factor increased from 92, in the absence of 1-octanol; to 148, in the presence of this alcohol in organic phase. The corresponding optimum concentrations of TOA in the solvent were 30 and 50g/l, respectively. The total separation of monocarboxylic acids from the mixture with succinic acid is possible by a multi-stage extraction process, adjusting the extractant concentration in each stage to that stoechiometrically needed for reactions with formic and acetic acids only. The addition of 1-octanol reduces the number of required extraction stages.