A PEG/Na2SO4 aqueous two-phase system (ATPS) was used for the extraction of butyric acid from a fermentation broth. Sodium sulfate was employed to precipitate the residual carbohydrate and nitrogen compounds, and then the filtrate from filtration was added into PEG to form two aqueous phases. By using response surface methodology (RSM), a study was made of the effects of PEG molecular weight and concentration, sodium sulfate concentration, temperature, pH on the extraction yield of butyric acid, to optimize the experimental conditions for the extraction. The study results showed that the optimal conditions for the extraction using PEG and Na2SO4 ATPS were 26% PEG 6000 (w/w), 9.6% Na2SO4 (w/w), 36 degrees C and pH 3.0, under which the yield of butyric acid was up to 91.74 +/- 0.46%, and the value was raised by about 3.7% through a two-stage extraction. Acetic acid was removed by distillation from the resultant mixed acids so that a relatively pure butyric acid was obtained. The present study proved it is feasible to extract and purify butyric acid from a fermentation broth by adopting the ATPS consisting of PEG and Na2SO4. (C) 2015 Elsevier B.V. All rights reserved.