The purpose of the presented study is to understand the physicochemical properties of proteins in aqueous solutions in order to identify solution conditions with reduced attractive protein-protein interactions, to avoid the formation of protein aggregates and to increase protein solubility. This is assessed by measuring the osmotic second virial coefficient (B(22)), a parameter of solution non-ideality, which is obtained using self-interaction chromatography. The model protein is lysozyme. The influence of various solution conditions on B(22) was investigated: protonation degree, ionic strength, pharmaceutical relevant excipients and combinations thereof. Under acidic solution conditions B(22) is positive, favoring protein repulsion. A similar trend is observed for the variation of the NaCl concentration, showing that with increasing the ionic strength protein attraction is more likely. B(22) decreases and becomes negative. Thus, solution conditions are obtained favoring attractive protein-protein interactions. The B(22) parameter also reflects, in general, the influence of the salts of the Hofmeister series with regard to their salting-in/salting-out effect. It is also shown that B(22) correlates with protein solubility as well as physical protein stability.