The erosion of three different polyanhydrides consisting of sebacic acid (SA) and 1,3-bis(p-carboxyphenoxy) propane (CPP) was investigated. Melt cast polymer matrices were prepared from the homopolymer p(SA) and two copolymers, p(CPP-SA) 20 : 80 and p(CPP-SA) 50 : 50. Particular attention was paid to the influence of the polymer matrix microstructure and of the monomers on erosion. Using polarized light microscopy we found that p(SA) and p(CPP-SA) 20 : 80 matrices consist of spherulites. SEM investigations showed that their crystalline parts are more resistant to erosion than their amorphous areas. The matrices erode into highly porous devices, whose porosity is detectable by mercury porosimetry. Using wide-angle x-ray diffractometry we found that monomers crystallize inside the pores. DSC investigations showed a maximum of crystallized SA after 2-6 days and a continuous increase of CPP, which stays in the devices for weeks. We conclude that the microstructure and the monomer properties are the two main factors which determine the erosion of these polymers. The obtained data on changes in porosity, crystallinity, polymer matrix thickness, erosion front velocities, crystalline monomer content, and monomer release provides the basis for quantitatively describing the erosion process.