The solubility of the hen egg-white lysozyme (HEWL) in aqueous solutions at pH 4.5 at various polyelectrolyte concentrations was first measured by a spectroscopical method from (278.15 to 298.15) K under atmospheric pressure. The employed polyelectrolytes were cationic polyacrylamide (CPAM), poly(ethylene imine) (PEI), poly(dimethyldiallylammonium chloride) (PDDA), poly(sodium styrene sulfonate) (PSS), and poly(vinylsulfonic acid, sodium salt) (PVS). The enthalpy and entropy of dissolution have been calculated, and the sol gel transition has been discussed as well.