AbstractThe in vitro degradation behavior of the polyester/polyether copolymer based on polycaprolactone/poly(ethylene oxide)/polylactide (PCEL) significantly depends on the caproxyl (CL)/lactyl (LA) ratio for the same poly(ethylene oxide) (PEO) content. The PCEL copolymer with rich CL units shows the crystalline structure of PCL with a slow degradation rate, while it presents the crystalline structure of poly‐L‐lactide (PLLA) with more LA units that has a fast degradation rate. In the case of PCEL with similar CL and LA unit contents, it only exhibits the crystalline structure of PEO and the degradation rate is faster especially during the early stage of the degradation due to the rapid cleavage of the PEO. The drug release system is prepared using 5‐fluorouracil (5‐Fu) as the model drug and PCEL copolymers as the drug carriers. It is shown that the drug release rate is controlled by the composition, morphology and properties of the PCEL copolymer. In particular, the PCEL copolymer with 70% LA units displays a near zero‐order sustained release in about 60 days. Therefore, these copolymers have potential application as a controlled 5‐fluorouracil delivery system. Copyright © 2001 John Wiley & Sons, Ltd.