The propionic acid is an important compound used in chemical and food industries as flavor precursor and for food preservation. The growing interest for green technologies applied to purification of biomolecules makes aqueous two-phase systems (ATPS) one of the most promising alternatives for extracting organic acids. The aim of this work was study the partition behavior of propionic acid in ATPS composed by polyethyleneglycol (PEG) 1500 g/mol and sodium sulfate. The effect of PEG (12.0 %, 13.5 % and 15.0 % w/w) and salt (12.0 %, 13.5 % and 15.0 % w/w) concentrations and pH (3.5, 5.5 and 7.5) on the propionic acid partition coefficient (Kp) was evaluated by a Face Centered Design, with 4 repetitions at central point and 2 replicates. Stock solutions of PEG 1500 (60% (w/w)) and sodium sulfate (20% (w/w)) at different pH values were prepared. ATPS of 25.0 g were prepared in centrifuge tubes by adding appropriates quantities of stock solutions of PEG, salt and distilled water. Tubes were centrifuged (2000 rpm/10 min) and incubated in a water bath at 25 °C for 24 h. Partitioning experiments were prepared in centrifuge tubes using 2.5 g of upper and lower phases of pre-equilibrated ATPS. Aliquots of 100 μL of pure propionic acid were added to ATPS and tubes were centrifuged and incubated in a water bath (25 °C/ 16 h). The phases were collected, total titratable acidity content was determined and the Kp values were calculated. Experimental results of Kp were submitted to the Analysis of Variance, and it was verified that Kp values increases as PEG and salt concentrations increases and pH is reduced. The highest Kp (50.20) was obtained for ATPS at pH 3.5 with PEG 15.0 % and sodium sulfate 15.0 %. ATPS is a viable alternative for the organic acids purification.