A method is described for calibrating quantitatively a temperature-programmed decomposition, mass-spectrometric (TPD-MS) system by monitoring the gases evolved during the thermal decomposition of a chemical within the TPD reactor. A method for calibrating for evolved CO and CO2 is described using the thermal decompositions of calcium carbonate and calcium oxalate. The method takes into account the production of CO+ ions from CO2+ ions and secondary reactions in the thermal decomposition of calcium oxalate.