Octanoated starch (OCST) with a high degree of substitution (DS = 2.7) is a fully amorphous and hydrophobic thermoplastic material. Its biodegradation was followed in activated sludge from a waste water treatment plant. Its blends with low-density polyethylene (LDPE) were also studied during soil burial for 6 months. From weight loss during the biodegradation period, it was found that OCST, even with such a high degree of substitution, is biodegradable. This was also verified with scanning electron microscopy. Holes were detected on the surface of the films as a consequence of starch consumption by microorganisms. Nevertheless, the rate of biodegradation is very small and depends on the amount of OCST in the blends. The mechanical properties such as tensile strength and elongation at break were measured. A reduction in both was found during the biodegradation period, mainly in blends with a higher amount of OCST.