Aqueous-aqueous polymer phase systems composed of polyethylene glycol 1000-potassium phosphate were used for the countercurrent chromatographic separation of proteins using the type XL cross-axis coil planet centrifuge. It was found that the peak resolution of proteins is highly dependant on the pH of the solvent system. The best separation of cytochrome c, myoglobin and human serum albumin was achieved with 16.0% PEG 1000-12.5% potassium phosphate buffer at pH 9.2. The resolution and separation time were improved by pH-peak-focusing countercurrent chromatography.