A comparative thermogravimetrie study of eight polyimidazopyrrolone films of varying composition revealed that the thermal stability of the polymers increased with increasing theoretical percentages of carbonyl groups within the polymer repeat unit. The films degraded more rapidly in air than in vacuo, but the order of stability was the same in the two environments. Dynamic heating of the film in vacuo developed three distinct temperature regions in which maximum rates of weight loss occurred at characteristic temperatures, and each temperature region was associated with the evolution of a particular gaseous species.