An apparatus designed for repetitive sampling and characterization of evolved gas mixtures generated during thermal analysis is described. The apparatus combines fast temperature ramp gas chromatography separations with mass spectrometric detection to selectively monitor volatile mixture component concentration changes as a function of sample temperature. The apparatus was tested by using it to repetitively sample and analyze the volatile products generated when poly(styrene) was catalytically cracked by an HY zeolite solid acid catalyst by heating the polymer/catalyst mixture in an inert helium atmosphere. Eleven mixture components contained in the gas stream were separated and detected at 90-s intervals when the polymer/catalyst sample was heated from 200 to 400 degrees C. Mass spectral extracted ion chromatograms were employed to generate species-specific evolution temperature profiles, which provided insight into thermal reaction mechanisms.