The surface properties of GroEL and GroES, typical heat shock proteins (HSPs) from E. coli, were characterized in terms of isoelectric point, surface net hydrophobicity and local hydrophobicity using aqueous two-phase systems (ATPS). The surface net hydrophobicities (HFS) of GroEL and GroES were found to be -360 and -222 kJ.mol(-1), respectively. This suggests that both HSPs are hydrophilic though GroEL has a greater hydrophilic surface than GroES. Both HSPs have some hydrophobic sites which bind Triton. The local hydrophobicity of GroEL is distinctly greater than that of GroES and also of most other water soluble proteins. GroEL and GroES have significantly different hydrophobicity patterns in respect to their surface properties and thus can be separated in the ATPS on the basis of these differences.