Two main-chain rigid-rod-like polyesters containing pendent t-butyl ester groups were synthesized. Simultaneous thermogravimetric analysis and Fourier-transform infra-red spectroscopy were applied to monitor the formation of isobutylene as well as other gaseous products that evolved at elevated temperatures. The polymers were also characterized by microscopy, differential scanning calorimetry and infra-red spectroscopy before and after thermal treatment.