A study was conducted to determine the degree of cyclization occuring in a series of four pyrrone and two polyimide prepolymers. These materials were heated from 25° to 400°C at 2°C/min in a helium atmosphere, and the effluent was sampled at regular intervals by a gas chromatograph/mass spectrometer technique. Quantitative analysis of this effluent indicated that the pyrrone polymers were less than 80% converted under these conditions while the polyimides were essentially fully converted. Significant amounts of carbon dioxide were found to be eliminated as the pyrrone polymers cured. This evolution was attributed to the loss of carbon dioxide from intermediate stages during cure and, in some cases, to the decarboxylation of unreacted groups. Support data obtained on model compounds are also presented.