Triblock copolymers consisting of a poly(dimethylsiloxane) (PDMS, Mn = 1000) middle block coupled to hydrolytically degradable polyester end blocks having different glass transition temperatures (T-g) as a newly developed biomaterial were synthesized by ring open polymerization initiated by both alcohol end groups of PDMS.H-1-NMR and X-ray photoelectron spectroscopy (XPS) were used to determine the bulk and surface compositions of the copolymers. Quantitative angle-dependent XPS data from cast thin films of the copolymers showed significantly high mole fractions of PDMS (0.7-0.92) at low takeoff angles as opposed to low mole fractions of PDMS (0.009-0.065) in the bulk. XPS data also showed that exposing the film to water significantly effect the surface composition of which PDMS concentration was decreased. This suggests that the copolymer chains undergo a structure rearrangement at the polymer water interface, depending on both T(g)s of polyester blocks in the copolymers and immersing time.