Novel surface biodegradable copolymers, poly(D,L-lactide-co-1-methyl-1,3-trimethylene carbonate) (PLMCA) and poly(D,L-lactide-co-2,2-dimethyl-1,3-trimethylene carbonate) (PLDMCA), have been synthesized by ring-opening polymerization with Sn(Oct)(2) as catalyst. The copolymers were characterized by H-1 n.m.r., C-13 n.m.r. and d.s.c. Water content and static contact angle of distilled water on the polymer surface were used to evaluate the hydrophobicity of the copolymers. Samples were implanted in rats to observe degradation characteristics. It was found that, in both the PLMCA and the PLDMCA copolymer system, the surface biodegradation characteristics in vivo were related to polymer hydrophobicities, which mainly depended on the copolymer compositions. The degradation of PLMCA and PLDMCA having a smaller ester fraction became a typical surface reaction. These copolymers may be useful in protein delivery systems.