Mass spectrometric thermal analysis (MTA), the determination of total ion current as a function of time and temperature, has been employed for investigation of pyrolytic reactions (I). In combination with differential thermal analysis (Ib) it has proved to be very useful for distinguishing between energy changes caused by phase transformations and those caused by decomposition reactions. Determination of total ion current alone is not particularly informative in most cases of polymer degradation, when several products may be formed simultaneously or sequentially. We have modified the usual procedure by repeatedly scanning spectra, as the temperature is raised on a linear program, and then plotting peak height as a function of temperature. MTA determinations conducted in this way have been helpful in several polymer investigations currently in progress in our laboratory. The alternative method, recording of a characteristic peak for each product on an individual channel of the Bendix Time-of-Flight mass spectrometer, was rejected as less reliable and more cumbersome when the number of products exceeds the number of channels on the instrument.