The acid-base surface characteristics of four polyesters: poly(L-lactic acid) (PLLA), poly(Dr-lactic acid) (PDLLA), polyhydroxybutyrate (PHB) and copoly(hydroxy butyrate-20% hydroxyvalerate)P(HB-20% HV) have been determined from contact angle and surface tension experimental data. Smooth surfaced polyester films were prepared by solution casting against a number of substrates ranging from high surface energy (aluminium, mercury, glass and freshly-cleaved mica) to low surface energy (poly(ethyleneterephthalate)(PET), poly(tetrafluoroethylene)(PTFE) and dry nitrogen gas). Results show that the acid-base interaction energy of the polyester surface is dependent on the casting substrate and ageing time. For a particular casting substrate, the equilibrium acid-base interaction energy between a polyester surface and an acidic liquid decreases in the order: PDLLA; PLLA; PWB; P(HB-20%HV). The time dependence of the acid-base interaction energy is interpreted in terms of orientation of surface acidic or basic sites. Furthermore, detailed results suggest that the initial acidic or basic character of the cast polyester surface is influenced by the acid/base surface properties of the casting substrates.