<p>The solubility of several water-soluble proteins, including hen egg-white lysozyme, bovine serum albumin, cytochrome C, and <i>Canavalia ensiformis</i> urease (from Jack bean), was examined in the presence of quaternary alkylammonium salts, tetramethylammonium perchlorate, tetraethylammonium perchlorate, tetrapropylammonium perchlorate, and tetrabutylammonium perchlorate. The quaternary alkylammonium cations markedly reduced protein solubility and were more potent than Hofmeister cations, such as sodium, potassium, and cesium. The quaternary alkylammonium cations reduced protein solubility mainly through the excluded-volume effect rather than electrostatic shielding or the hydrophobic effect. This result highlights the importance of the excluded-volume derived from salts themselves in the salting-out effect and provides insights into the mechanism of the Hofmeister series.</p>