Background: Aliphatic polyesters are widely used for drug delivery applications, including the archetypical poly (lactic-co-glycolic acid) (PLGA). The main drawbacks of polyester-based particles are initial burst release and acidity at the site of particle degradation. A novel polyester; poly (glycerol adipate-co-w-pentadecalactone) (PGA-co-PDL), was designed and synthesized to overcome these drawbacks. The study aim was to evaluate the stability and degradation of polymeric microparticles (MPs) prepared from PLGA and PGA-co-PDL under in-vitro conditions relevant to those encountered in the lungs, i.e. as suspension in simulated lung fluid at 37oC under axial rotation 15 rpm/ minute. Methods: MPs were formulated from PGA-co-PDL and PLGA by a single emulsion method using Poly vinyl alcohol (PVA) as an emulsifier. The MPs were characterized over 28 days for size, morphology and degradation features using scanning electron microscopy, charge using Zetasizer ZS series, pH by pH meter, and molecular weight using gel penetration chromatography (GPC). Results: Over time, PGA-co-PDL MPs reduced in size, showed observable degradation characteristics and increased suspension acidity more slowly than PLGA MPs. A reduction in molecular weight over time confirmed that PGA-co-PDL MPs were slower degrading than PLGA MPs. The trend over time indicated PGA-co-PDL MPs were more stable than those formed from PLGA. Conclusion: PGA-co-PDL MPs carriers were more stable than PLGA MPs under the investigated experimental conditions. They are promising carriers for sustained release formulations.