The need to high quality therapeutic proteins has grown significantly in the past 30 years. Recombinant proteins are often produced from vectors inserted into E. coli cell lines for large scale production. However, over-expression of the protein within the cell can lead to the formation of large, insoluble aggregates known as inclusion bodies. Native monomer protein can be isolated from inclusion bodies through a refolding process. This entails disruption of the aggregate structure with high concentrations of denaturant and renaturation in native-promoting solution. Our work characterises protein-protein interactions and aggregation between partially unfolded proteins during the refolding process. The protein-protein interactions are characterized in terms of the osmotic second virial coefficient (B22). A positive value indicates repulsive interactions while a negative value indicates attractive interactions. Measurements are carried out for lysozyme, ribonuclease A and preproinsulin as a function of pH, ionic strength and denaturant concentration, alongside a range of known refolding excipients.Past studies (Ho and Middelberg, 2004; Ho et al., 2003) have shown a link between higher B22 values in denaturant solutions and reduced aggregation during refolding. Our experiments have focused on the effects of urea and GdmHCl upon protein-protein interactions, alongside how ionic strength and refolding additives influence interactions between partially-folded states. At low ionic strength, solutions of urea increase net repulsive interactions compared to GdmHCl solutions through an attenuation of short-range attractive interactions. Electrostatic repulsive interactions are screened in solutions of GdmHCl due to the increased ionic strength of the solution; however short-range attractive interactions are also attenuated in a similar fashion to urea solutions. Protein-protein interactions in low and high concentration denaturant solutions have been shown to be highly sensitive to ionic strength and refolding experiments have shown that this correlates with increased aggregation during refolding. The solubilising additive Arg HCl has been shown to reduce short-range attraction between proteins in urea solutions, while the folding-promotor additives sucrose and hexylene glycol have been shown to have a more complex effect on protein-protein interactions in urea solutions dependent on denaturant concentration.Within the wider context of the field of protein aggregation and refolding, the work conducted here will contribute towards the understanding of how denaturants and solutes influence attractive protein-protein interactions and aggregation behaviour between unfolded or partially folded proteins.