The decomposition mechanism of tetrapropylammonium (TPA þ ) template cations in several commercial ZSM-5 precursors was studied by temperature-programmed desorption/oxidation using a mass spectrometer (TPD/TPO-MS). Different oxygen concentrations, gas flow rates and temperature ramp rates were examined to understand the effects of these factors on the organic template decomposition. The TPA þ template decomposition pattern is strongly dependent on the SiO 2 /Al 2 O 3 molar ratio (SAR) and the oxygen concentration. The TPA þ template cations may be associated with three different sites depending on the SAR: Lewis acid sites (surface Al-O À ), Brønsted acid sites (framework Al-O À ), and weakly acidic BSiAO À sites. The thermal decomposition of TPA þ templates at different sites produced different products, including water, ammonia, propylene, ethylene, methane, and n-and iso-propylamines. However, no significant heavy volatile compounds were detected, suggesting that secondary reactions (e.g., oligomerization of olefins) were extremely limited and negligible in a fixed-bed micro-reactor used for TPD/TPO. The TPD-MS results suggest that the thermal decomposition mechanism on Lewis acid sites is possibly a free radical type of reaction, while on Brønsted acid sites and BSiAO À sites, the decomposition undergoes both Hoffman elimination reaction and a radical type of reaction. The presence of oxygen significantly lowered the TPA þ decomposition temperature, which is also a function of oxygen concentration. The knowledge about the template decomposition pattern and the composition of the evolved volatiles obtained by the TPD/TPO-MS technique may be used to develop ideal organic template removal procedures for zeolites.