Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR) were used to determine the relative heat of fusion, crystallinity, and composition of thermo-oxidized low-density polyethylene (LDPE) inoculated with Phanerochaete chrysosporium and incubated for 3 months at 30 degrees C under nonaseptic conditions. DSC analysis of thermo-oxidized samples (150 degrees C, 120 h) followed by microbiological treatment showed a remarkable reduction on the heat of fusion (36%) compared to that of untreated LDPE. TGA allowed determination of changes due to the biological activity but not those that occurred by thermal treatment. A significant increase in the carbonyl index of the thermally treated (150 degrees C, 120 h) LDPE samples was observed due to the increase in the oxidation level during this treatment. In contrast, the double-bond index (evaluated by FTIR) increased considerably (23%) as a result of the microbial treatment. The observed structural and morphological modifications of the biologically treated LDPE indicate that the reduction of the chain size may be carried out by scission mechanisms.