Abstract: In this paper, tri-octylamine (in octanol) was selected as extractant to extract lactic acid and acetic acid. The effects of the extractant content and the pH on the extraction equilibrium have been investigated for lactic acid, acetic acid and their mixture. The models of the distribution coefficient were proposed for the single component and the mixture based on some assumptions. At the conditions of the experiment, the distribution coefficients for the single acid and their mixture and the separation factor increased with the content of the extractant, and decreased with the pH. The distribution coefficient models fitted in well with the experimental data, and the complexation reaction constant K 11 for single acid system can be used to predict the distribution coefficient of the mixture.