The methanolysis of poly(ethylene terephthalate) (PET) copolymers containing 5-nitroisophthalic units was investigated. Random copolyesters containing 10 and 30 mol % of such units were prepared via a two-step melt copolycondensation of bis(2-hydroxyethyl) terephthalate (BHET) and bis(2-hydroxyethyl) 5-nitroisophthalate (BHENI) in the presence of tetrabutyl titanate as a catalyst. First, the susceptibility of these two comonomers toward methanolysis was evaluated, and their reaction rates were estimated with high-performance liquid chromatography. BHENI appeared to be much more reactive than both BHET and bis(2-hydroxyethyl) isophthalate. The methanolysis of PET and the copolyesters was carried out at 100 degreesC, and the degradation process was followed by changes in the weight and viscosity, gel permeation chromatography, differential scanning calorimetry, and H-1 and C-13 NMR spectroscopy. The copolyesters degraded faster than PET, and the rate of degradation increased with the content of nitrated units. The products resulting from methanolysis were concluded to be dimethyl terephthalate, dimethyl 5-nitroisophthalate, ethylene glycol, and small, soluble oligomers. For both PET and the copolyesters, an increase in crystallinity was observed during the degradation process, indicating that methanolysis preferentially occurred in the amorphous phase.