Two new copolyesters consisting of an alternating disposition of glycolic acid units and 4-hydroxybutyric or 6-hydroxyhexanoic units were assayed in the form of microspheres as drug delivery systems. Despite their similar chemical constitutions, great differences in properties such as the melting point and glass-transition temperature were found between the two polymers. Microspheres could be successfully prepared by the oil/water emulsification/solvent evaporation method and revealed different morphological features depending on the polymer matrix. The encapsulation and release of triclosan in a cell medium and in a Sorensen solution were evaluated. The release profiles, which were clearly different in both polymers when the hydrophobic cell medium was employed, indicated that the release reached an asymptotic value that was always lower than the maximum release expected for the drug loading. No significant degradation occurred during drug release. The charged microspheres of both polymers were highly crystalline, and the incorporation of drug particles into the polymer crystalline domains was feasible. No trace of single triclosan crystals was found in differential scanning calorimetry or X-ray synchrotron experiments. (C) 2008 Wiley Periodicals, Inc.