Two different polyethylene glycol (PEG) - salt aqueous two-phase systems (ATPSs) were studied, at T = 298.15 K. Different polymer molecular weights, 4000 and 6000, were used to form the biphasic systems. The binodal curves and the tie-lines were determined using analytical methods commonly studied in the literature, such as cloud point method, conductivity and gravimetric analysis. The partition coefficients of four DNP-amino acids, in six different tie-lines, were determined spectrophotometrically. Studies on the behaviour of (liquid-liquid) phase equilibrium and partitioning showed that, in the range of the studied polymer's molecular weight, the system formed by PEG with higher molecular weight presents higher efficiency. The influence of PEG's molecular Weight on the partitioning process was considered in terms of the relative hydrophobicity of the phases, using the Delta G*(CH2) calculations. (C) 2016 Elsevier B.V. All rights reserved.