A series of liquid polyester polyols from adipic acid (AA), phthalic anhydride (PA), ethylene glycol, propanediol-1,2, and trihydroxymethylpropane, varying in the molar ratio of PA to AA, were prepared. The effects of the o-aromatic ring in the molecular chain, which came from PA, on the viscosity, glass-transition temperature, and thermal degradation temperature of the polyester polyols were studied with viscometry, differential scanning calorimetry, and thermogravimetry. The intrinsic viscosity and glass-transition temperature increased with the concentration of the o-aromatic ring increasing. The temperature of the maximum thermal degradation rate for aliphatic polyester polyols was 434.20 degreesC. Two steps of thermal degradation were found when there were o-aromatic rings in the molecular chain. One thermal degradation temperature was 358.36-360.48 degreesC, and the other was 412.85-427.18 degreesC. Polyester polyols with o-aromatic rings had higher stability at lower temperatures (<240.00C). However, aliphatic polyester polyols had higher stability at higher temperatures (300.00-480.00 degreesC). The activation energy and order of degradation were calculated from thermogravimetric curves.