The influence of chloride salts of Na+, Rb+ and Cs+ at concentrations from 0.15 to 1.2M was studied with bovine albumin, trypsin, ovoalbumin and lysozyme partitioning in an aqueous two-phase system formed by polyethyleneglycol 1500 and potassium phosphate at pH 7.4. Monovalent cations favoured the protein transfer to the polyethyleneglycol rich phase in the following order: Rb+ > Na+ > Cs+. Structure making cations as Na+ induced a poor loss of structured water, producing little diminution of the molar partial specific volume of polyethyleneglycol, while Rb+ and Cs+, structure breaking cations, induced a significant decrease in the specific volume of the polyethylene glycol. The increase of available solution free volume in the top phase favours the protein transfer to the polyethyleneglycol rich phase. Na+ and Rb+ induced a slight decrease in the alpha helix content of the proteins, while Cs+ increased the secondary structure for all the proteins. All the cations induced a decrease in the hydrophobic surface of the proteins, this effect was more significant in the presence of Cs+.