Proteins tend to undergo irreversible inactivation through several chemical modifications, which is a serious problem in various fields. We have recently found that arginine (Arg) suppresses heat-induced deamidation and beta-elimination, resulting in the suppression of thermal inactivation of hen egg white lysozyme and bovine pancreas ribonuclease A. Here, we report that poly(ethylene glycol) (PEG) with molecular weight 1,000 acts as a thermoinactivation suppressor for both proteins, especially at higher protein concentrations, while Arg was not effective at higher protein concentrations. This difference suggests that PEG, but not Arg, effectively inhibited intermolecular disulfide exchange among thermally denatured proteins. Investigation of the effects of various polymers including PEG with different molecular weight, poly(vinylpyrolidone) (PVP), and poly(vinyl alchol) on thermoinactivation of proteins, circular dichroism, solution viscosity, and the solubility of reduced and S-carboxy-methylated lysozyme indicated that amphiphilic PEG and PVP inhibit intermolecular collision of thermally denatured proteins by preferential interaction with thermally denatured proteins, resulting in the inhibition of intermolecular disulfide exchange. These findings regarding the different mechanisms of the effects of amphiphilic polymersPEG and PVPand Arg would expand the capabilities of methods to improve the chemical stability of proteins in solution. Biotechnol. Bioeng. 2012; 109: 25432552. (c) 2012 Wiley Periodicals, Inc.