Abstract The partition coefficients of lysozyme, chymotrypsinogen A, albumin and catalase in 64 polyethyleneglycol/dextran systems are reported. The measurements were performed at four pHs for each protein. The simultaneous effect of pH changes and polymer molecular weight and concentration on the partition coefficient of each protein is analyzed. The partition coefficient of lysozyme (IP = 10.5) has a minimum value at its isoelectric point and it takes its maximum value at acidic pHs. We observe a change in the aggregational state of lysozyme when the pH is shifted from the acidic to the alkaline ranges. The partition coefficient of chymotrypsinogen A (IP = 9.5) has a minimum at pH 5.6 and increases towards more alkaline or acidic pHs. The partition coefficient of albumin (IP = 4.6) takes its minimum value at pH 5.6. The partition coefficient of catalase (IP = 5.6) takes its maximum value at pH 5.6. The effect of the pH on the partition coefficient of lysozyme and chymotrypsinogen A at high polymer concentrations is larger than at low total polymer concentrations but the effect of the pH on the partition coefficient of albumin and catalase at high polymer concentrations is smaller than at low polymer concentrations. We find that the partition coefficient of the four proteins studied becomes less sensitive to changes in the pH at high PEG molecular weights. Close to the isoelectric point the partition coefficient is less sensitive to changes in the molecular weight of the polymers than at conditions far from the isoelectric point.