The effect of mobile phase pH on the retention characteristics of eleven proteins was examined in hydrophobic interaction chromatography (HIC) on a SynChropak propyl stationary phase. Selectivity was shown to change with eluent pH. The effect of the displacing salt on the separation of proteins on a weakly hydrophobic weak-anion-exchange chromatography (AEC) packing was examined. Some differences in selectivity were observed when sodium sulfate was used as the displacing salt, compared to that observed with sodium chloride in the eluent. It was demonstrated that these AEC packings exhibited both electrostatic and hydrophobic properties, depending upon the type and concentration of salt used in the mobile phase. The addition of 20% ethylene glycol to the mobile phase was shown to reduce the hydrophobic interactions. The application of weakly hydrophobic weak-cation-exchange packings to HIC of proteins was demonstrated. Elution of such columns with descending sodium sulfate gradients was found to provide a selectivity different from that observed with a propyl stationary phase. Manipulation of mobile phases was shown to provide useful selectivity as a result of the combination of electrostatic and hydrophobic contributions to the separation process.