Poly(glycerol adipate-co-o-pentadecalactone) (PGA-co-PDL) was previously evaluated for the colloidal delivery of a-chymotrypsin. In this article, the effect of varying polymer molecular weight (MW) and chemistry on particle size and morphology; encapsulation efficiency; in vitro release; and the biological activity of a-chymotrypsin (a-CH) and lysozyme (LS) were investigated. Microparticles were prepared using emulsion solvent evaporation and evaluated by various methods. Altering the MW or monomer ratio of PGA-co-PDL did not significantly affect the encapsulation efficiency and overall poly(1,3-propanediol adipate-co-o-pentadeca- lactone) (PPA-co-PDL) demonstrated the highest encapsulation efficiency. In vitro release varied between polymers, and the burst release for a-CH-loaded microparticles was lower when a higher MW PGA-co-PDL or more hydrophobic PPA-co-PDL was used. The results suggest that, although these co-polyesters could be useful for protein delivery, little difference was observed between the different PGA-co-PDL polymers and PPA-co-PDL generally provided a higher encapsulation and slower release of enzyme than the other polymers tested.