The nature of the solid-liquid interface of hydroxyl-terminated hyperbranched aromatic polyester (HPB-OH) thin films has been studied with respect to further protein adsorption investigations. Contact angle measurements, atomic force microscopy, zeta potential measurements and spectroscopic ellipsometry were used to characterize the physical and chemical structure of the HBP-OH thin films as well as the adsorption-relevant surface properties.It was shown that film properties such as the effective refractive index significantly depends on the thickness of HBP-OH films whereas the water contact angle does not reveal any changes with thickness variations. All surface-sensitive methods used indicate marked swelling behaviour of HBP layers in aqueous phosphate buffer saline. Moreover, the degree of swelling, zeta potential as well as surface free energy (in terms of water contact angle) strongly depend on the time the films had been annealed above their glass transition temperature. (c) 2005 Elsevier B.V. All rights reserved.