The erosion mechanism of a class of bioerodible polyanhydride copolymers consisting of a fatty acid dimer (FAD) and sebacic acid was investigated in vitro. Special attention was given to the behaviour of the monomers that are released from the polymer matrix during the degradation process. The erosion of polymer matrix discs was followed by polarized light microscopy, differential scanning calorimetry, wide-angle X-ray spectroscopy and Fourier-transform infra-red spectroscopy. FAD, which is derived from erucic acid, was found to decrease the surface tension of the buffer significantly. Owing to its low solubility, however, as the polymer degrades, most of the FAD is deposited on the surface of the polymer matrix discs. There it forms an amorphous monomer film consisting of a mixture of FAD and FAD salts. The acid/salt ratio is pH-dependent and changes during erosion. Such films behave as diffusion barriers when these polymers are used as materials for the release of drugs from implants.